I am at uni atm, but i am in a scholarship scheme set up by the RAF to become a pilot, so i kinda know what it is like to be away from your gf (in my case) but bf in yours for a little while.
Firstly, don't whatever you do, seriously don't try and persuade him not to join the army, if its a life long dream even the notion of you saying i don't want you joining and leaving me is firstly very selfish on your part and secondly if i had my gf say that to me, i would run so fast you wouldn't see me leave. Don't tell him not to do his dream, that is number 1.
Secondly, the forces are not as bad as they seem, I went for IOT (Initial Officer Training) over the summer at RAF Cranwell over the summer which is a 12 week intensive course which basically teaches you discipline, guns, etc...during this time you are not really allowed to see anyone else and certainly when i went it was very much a 'closed camp' however this does not mean i didn't stay in touch, we had email and telephones to call our parents and gf from nearly every day (unless we were camping out in the middle of nowhere...!) so although you don't see your loved ones for a long and arduous 12 weeks you still talk to them and tbh i you don't see each other very much atm i doubt it would be much different.
I don't know what the army training is like, but if he wants to be an officer he will also go through IOT, if he wants to be a cadet i think it is a 6 week initial course but an 18 month training period? I can't remember of the top of the head and am basing most of it on what the RAF Regiment have to go through.
As a whole the forces understand the need for soldiers to have a physical contact with a member of the opposite sex, hell its a great motivator and improves morale, a night with a soldiers wife is equal to no other morale boost if you get my drift, but at the same time your bf will be in full term employment and his main duties will be with HM most of the time.
He will still see you, of course, but you have to realise that he has other duties which he took an oath to fulfil.
Graham