edmc

Hi OP! 
just in case you wondered how to get married in Canada!
Marriage in Canada on Visitor Visa
But otherwise you are free to marry in USA as long as your Canadian boyfriend returns home!
We did the K1 as a US/Canadian couple and honestly I'd recommend anyone to go the CR1 route instead of K1 especially if your partner is Canadian. K1 isn't that fast anymore. Might still be of use for people from certain countries or backgrounds but for a Canadian partner I would recommend CR1
 
Good luck!

Every couple has their own priorities, and each couple must decide which visa is better for their situation.
K-1        
    More expensive than CR-1    
    Requires Adjustment of Status after marriage (expensive and requires a lot of paperwork)    
    Spouse cannot leave the US until she/he receives approved Advance Parole (approx 6-8 months)    
    Spouse cannot work until she/he receives EAD (approx 6-8 months)    
    Some people have had problems with driver licenses, Social Security cards, leases, bank account during this period    
    Spouse will not receive Green Card for many months after Adjustment of Status is filed.
    A K-1 might be a better choice when 18-21 year old children are immigrating also
    In some situations, marriage can affect certain Home country benefits, making a K-1 a better choice   
    A denied K-1 is sent back to USCIS to expire
    
CR-1
    Less expensive than K-1    
    No Adjustment of Status (I-485, I-131, I-765) required.    
    Spouse can immediately travel outside the US    
    Spouse is authorized to work immediately upon arrival.    
    Spouse receives Social Security Card and Green Card within 2 or 3 weeks after entering the US    
    Opening a bank account, getting a driver's license, etc. are very easily accomplished with GC, SS card, and passport.
    Spouse has legal permanent Resident status IMMEDIATELY upon entry to US.
   

 

The substantial presence test relates to taxation, not immigration.  It means he must file a US tax return if he stays in the US for too long, as defined by IRS here -- https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
 
Even if your bf stays less than 6 months in the US, if he stays for months at a time or visits too often, the CBP officer at the border may suspect that he is actually living in the US rather than just visiting.  That could result in your bf being denied entry at the border.  Or worse, the CBP officer might suspect that your bf is working illegally while in the US.  Remember that he cannot work, not even remotely for a non-US company, while in the US as a tourist.
 
 
It is perfectly legal for you and your bf to get married in the US if he returns to Canada soon after.  I got married in the US while on a visit, then returned to my home country on the date in my original return ticket.  The US government was fine with it, and now I have my green card.  I'm not sure about the marriage requirements in Canada, but you could go there to marry (instead of marrying here), then file your petition to start the immigration process for your spouse.
 

If your divorce is final and he was never married before or his divorce is also finalized, you're free to marry.
People from all around the world get married in the US on tourist visas to leave the country soon after that. You're not breaking any laws by marrying. The only thing is, CBP may ask about the purpose of his trip and he cannot lie. Also, don't plan adjusting his status after marrying, as this is fraud if planned ahead before he entered the US.

The rules say 2 years between I-129F petitions.  If you can't wait 2 years, you may submit a waiver request to allow you to file another petition.  But that doesn't apply in your case as it has been more than 2 years since you filed the I-129F for your ex.  So if you want to file a petition for your bf now, you may do so without needing a waiver.
 
That said, if/when you feel ready to marry your bf, consider marrying and pursuing a spouse visa instead.  Your bf has easy access to come visit anyway while waiting for the visa process to complete.  The timeframe for K1 processing is much longer now than it was back in 2017, and for some lucky couples, CR1/IR1 spouse visa processing turned out to be even shorter.
 

+100 to this. The K1 processing times nowadays are almost as long as your first marriage lasted.