Didn't read most of this thread, but I have personal experience in this matter so I'll throw in my thoughts (though I agree completely with
@mimelim's post).
Most long-distance relationships that were going on in my med school class ultimately failed, and pretty quickly. There's only one guy in my class that I can think of whose long-distance relationship survived the gauntlet of medical school. Most crashed and burned within the first 6 months of medical school and the remainder by the end of first year. I don't tell you that to discourage you - just to be realistic.
It's tough because you will likely grow close to some of your classmates. You will share that experience - your frustrations, feelings of defeat, feelings of accomplishment, commiseration, etc. - with them that you can't necessarily share at the same level with your SO, unless he's also in med school. Hell, this is true even if you're living with your SO. I imagine it can be difficult to stay connected when the main thing that you're doing takes a bunch of your time but you're unable to share that experience, at least superficially, with your SO. On the flip side, you glow close to the people that you do share that experience with, which might make you want to be with some of those folks...
I'm not a fan of long-term relationships, but if you'd like to give it a shot then go for it. I would just be realistic. I think it's an easier thing to do in med school than in college since you have theoretically matured a bit. But based on my experience the odds are stacked against you. It can work, but you need to be dedicated to the relationship first and foremost. You and your SO need to understand that it will be difficult at times to have long conversations or visits depending upon how busy you are with school or how busy he is with whatever he's doing. You need to understand and expect that unless you're an academic freak, the vortex that is med school will pull you away from just about anyone that isn't right in front of you, at least until you get your feet on solid ground and get with the groove. You both need to accept that and be willing to deal with it - something that I've found the partner that isn't involved in medical training is ok with initially until they see how it works in reality.
Good luck.