I have a lot of experience with this kind of decision making.

So, if you were my best friend, here's what I'd tell you: Don't do either thing. I've been in your shoes. I've been 21, in an amazing relationship with a guy I was crazy for, the two of us trying to plan two separate careers that pulled us in different directions. So my best advice, my honest advice, is to do nothing. For now.

First, don't go to get a D.O. That's kind of useless. You want to be an M.D. and you need to do that. He, frankly, shouldn't get a D.O. but that's his choice.

What I'd do, if I were you, is postpone going to school for a year. Live together, enjoy a year off, have a lot of sex, work a job that pays the bills, put some savings in the bank. Then, once you've graduated, make a new plan to accommodate both of you. Based on where you get accepted and where he gets accepted and what you both want to do, find a city that offers BOTH of you what you want. You can also spend that year getting pretty certain of this relationship (or not) which will save you both from tying yourselves to a situation that may or may not work out.

If he doesn't have the cred to get into med school proper, find him a program in a city that DOES have a Med school for you so you can live together, remain together, but both pursue careers at the top of your game. Hell, if the MCATS were his limiting factor, he might try those again and see if he can't clear the bar to join you at a more competitive program.

It is only a year. That's nothing. It gives you a year to test the waters of this amazing guy, it gives you a year to get a little cash reserve under yourself, and it gives you an extra year to plan.

That's what I'd tell my best friend to do, anyway. (PS: Just got engaged to that guy I took a gamble on at 21. I am so fucking happy right now I could jump up and down. There are no guarantees, but I never regretted it. I also never compromised my vision of my future for him, either. We took it slow, we played it strategically, we made decisions together that worked for BOTH of us or not at all. That's how this gets done, in my opinion).