Sorry to be the bearer of negative news, but this is what I've accumulated as collective data concerning long term relationships over the years I've heard girl friends upon girl friends upon girl friends yacking on to poor ol me about.
If you guys started off as internet friends or something of the sort with minimal personal contact. You would have very poor chances, it's hard trying to be committed, when it feels like you are single most of the day except when you are on the phone/online with that person. Assuming you guys are of a younger age range, you'd be having tons of opportunity seeing new people.
If he is around the age where he starts college, or enters college, there is also a minimal chance of things working out. Pretty much for the same reason as above, he is in a place where he has the opportunity to meet hundreds of girls, its going to be tough just trusting him. Being a college student myself, I've known quite a few number of girls who had their bfs (fairly recent relationships) go far away for college, I can't recall any of them still being together.
I only know about one or two couples that were high school sweet hearts and still remain together despite having started off in different colleges.
This is college life however, I don't know what your situation is.
If you guys are adults, have careers and have mentally settled down in the exciting world of relationships, have been together for quite a number of years, then I would not be surprised if you guys can beat the distance.
Don't dwell so much about the future, if it doesn't work out, that is just fate, the best thing you can do is try to enjoy every bit as possible and make sure that in the future, even if you are no longer with him, you can look back at those memories with a smile on your face.