Hi Jessie Jayne,
In your letter I would just say that you are currently living in DR with your bf, and leave it at that. Also, maybe submit a copy of the rental agreement, or pay stubs of the apartment you have rented there, with both of your names on it. The more proof u submit that you're here in DR, the better. If you draw attention to the fact that you will be remaining in Canada, that will only work against you. You should definitely not lie on the application, because that will come back to bite you in the butt, but just make them understand that upon submitting the applciation, you're living here in DR, and just leave it at that.
Also, if you can get additional proof of finances, whether it be from your dad... or get your dad to also write a letter of invitation inviting your bf to come and stay in his home (therefore, accommodations paid for and won't be an additional expense).
You do not need to have the letter of invite sent from Canada. When my bf got his visitors visa in October, my parents wrote the letter of invitation, since we were going to be staying at their house over the christmas holidays, and then I just had them sign it and scan it into the computer, so my bf and I could print it from here in DR, and it also had their signatures on it....
Your boyfriend should submit a letter from his employer stating that he can have the days off of work that he has asked for and that he understands that he is expected back at work on such and such a date....
It really is a gamble, but one thing is for sure: if you don't try, you definitely won't get it. So good luck.
Again, submit the application WHILE you are still living in DR. I believe that reason is why my bf got the visa... with proof that I live and work in DR.