llaz

If you husband files now, he files as an LPR.  Once he naturalizes he notifies either USCIS or NVC, as applicable to "upgrade" from LPR to US Citizen.  This removes any quota related delays.

If you husband files now. He will be filing as a green card holder (F2A) visa . Once he becomes a US citizen he will inform USCIS or NVC he is a citizen and the petition will change from F2A to IR2. 
Right now only you can file a petition for IR2.
I want to ask again. Does your stepson want to stay in the US or does he want to go home while the process takes place
and 2. Is the mother of your stepson okay with him moving to the US? 

First, whether you have any relationship with him or even ever met him is not relevant to getting his IR2 visa.  Don't know where anybody got that idea.  The bona fide relationship issue is only related to the spouse, not stepchildren.
 
Your husband can file now and upgrade his petition when his Naturalization is complete.  Sponsorship comes at the NVC stage, so he can sponsor his son himself.  That seems the best option to me.
 
You could petition the son, and your husband would be your "household member" when it comes to sponsoring.
 
Another option is for you to make a concurrent filing for the son to adjust status NOW, and stay here.  Then he can become a citizen once he has a green card and your husband naturalizes.  Might be some timing issues there, but citizenship cancels any sponsorship obligations.  Of course, he's still your husband's son, so real life responsibilities come into play.
 

Naturalising varies by office, 6 months may be an average.

He will need travel insurance from his home country. Travel insurance will cover the cost of taking him back home if he is hospitalized here and misses his booked flight home and needs to buy a new ticket or if he needs to be taken home with medical assistance. Your health insurance would not cover that. 
*~*~*question regarding visiting moved to “tourist visas”*~*~*

No.
 
You can purchase travel insurance for him from within his country before he travels.  Before my husband emigrated, he had a NZ-based insurance plan that covered multiple trips over a one year period.  

Hey all! About had a heart attack with all your talk on the first page about a new form 😂😂 but apparently the new form still hasn’t been uploaded, thank goodness since I downloaded the form early Jan filled it out and then started gathering documents. Sent in our form and docs on Saturday, 2 day delivery, was delivered today at Phoenix. Excited to be done with the paperwork gathering part I feel like USCIS paperwork has been our life for the past 4-5 weekends. 

Just got my i-797 NOA today in the mail. Received date 2/10, Notice date 2/12. Now just gotta sit tight and not think too much about this. 

***Update***
02/08: roc sent
02/10: received in Phoenix
02/14: check cashed
 

Or you can submit an I-130 to petition for your step-child; see "Step-parent" row: https://www.visajourney.com/guides/us-immigration-guide-for-children/

Is there any issue with traveling just before the 90 days would start for ROC?
My husband's mother is dying and his conditional green card expires early April. He wants to go see her in December. 

There are no issues traveling 

No issues on traveling.

Sorry about mom.
 
It's safe for him to travel in December.  Make sure he returns before his green card expires. 

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

Hello Missileman and Everyone, 

Just wanted to let you all know and give you an update. The paper was in fact an interview notice for AOS and we were verbally accepted by the immigration officer! We still are blown away by how quickly the process went for us being that we waited for so long for K1. 

Thank you all for the support and guidance. 

We had our interview today with USCIS for AOS, we were approved! 
Very excited!

Our interview letter came on just a regular printed letter, no fancy NOA or anything.

The immigration officer was firm but friendly. She asked a lot of dates questions to my husband (birthdays, his mom's birthday, wedding date, when you entered into the US last, how many times etc.) not a lot of questions at all about our relationship or marriage. She asked for proof of our marriage we brought essentially everything that's needed for ROC that we had as of right now with both of our names (bank statement, health insurance cards, car title, car insurance, house offer that we just did on Saturday, preapproval notice from bank etc.) and photos. She was a bit frustrated that I didn't bring my 2017 tax documents or my original birth certificate. She asked my husband all of the questions, my husband and the translator sat in the chairs closest to her desk and I sat in the chair behind them, I was asked no questions at all (I am the USC). She said that it was very clear our marriage is legitimate and that we were approved.  

Received exactly the same letter last year. Was a regular i751 interview. 

Sounds like a regular interview letter to me.

We had an AOS interview, too. Just follow instructions. Don't worry; you'll be fine. 

I found a discussion about this, and if you look at the bottom, it was not a stokes interview for them.
 
http://www.trackitt.com/usa-discussion-forums/i130-i485/1003462125/is-form-g56-letter-a-stokes-interview-notice