Renna

If he is approved, he will be issued a Identification Certificate and be eligible for a PH passport, http://consular.dfa.gov.ph/passport/10-passport/61-passport-requirements-new-adult:

@Renna, if your husband is a dual US and Philippine citizen he will need both a US passport and a Philippine passport; Philippine passport in order to leave the Philippines and US passport to enter the US: https://www.theworldwasherefirst.com/travelling-with-two-passports/ If he is only a US citizen, then he only needs a US passport.

Thank you so much!

1. He has to file the last three years of tax returns to the IRS, even if he filled and paid taxes in the Philippines, and even if he does not owe the IRS any back-taxes.  These will be needed in the CR1 process.  If he has already done this, great.
2. He needs proof of US citizenship--a document like a birth certificate or US passport.
3.  He will need some kind of travel document like a US passport for international travel.
4.  Once he has #2, he can file the I-130 petition on your behalf, together with evidence of your bona fide relationship and a marriage certificate.
5.  While you are waiting for the I-130 to be approved (7-9 months), he can leave for the US, find a job and start earning sufficient income and a place to live.  The income and US domicile will be needed for the NVC stage of the CR1 process.  You stay in the Philippines and wait unless you want to visit, and in that case you apply for a tourist visa but those are rarely approved when a CR1 is in process.
 
You could also look into DCF, but he will still need a job in the US with sufficient income to support both of you and a US domicile.
 
Good luck!

Has he been filing his US taxes since he began working as all US citizens living abroad are supposed to do? If not, he has some backfiling to do.

No no, nothing like that, but there is a process. You should research into DCF requirements for the embassy in the Manila.
 
One big one is domicile, so since he has never lived in the US, he needs to establish something there.
Also finances, he has to be able to prove how he would support himself and you there in the US, I'm not super familiar with this for DCF but assume it's the same as with CR/IR-1's using the I864 form.
 
Also, with DCF, he must have some permanent residency status in the country he is at, being the Philippines but since you say he has been there all his life basically I assume he does.
 
Again I would look into that if I were you. 
 
And as @databit mentioned, he will need to clear up some stuff with the IRS. I assume probably has not be required to file taxes but he has deal with it regardless by at least specifying he wasn't required to. Definately need to get this cleared up.
 
Here's a few links with some info:
 
https://www.uscis.gov/about-us/find-uscis-office/international-offices/philippines-uscis-manila-field-office
 
http://visaharmony.com/dcf.php
 
 

Would be a spousal or CR-1 visa. K-3 is basically obsolete.
 
Yes, all paperwork can be done from abroad... but, DCF is the quickest route honestly. Easily the fastest.
 
The main issue is he will have to prove domicile in the US (bank account, property/rental, job, ect) before hand. Since he has never been to the US just moving there with you isn't just like getting on a plane.