Being overseas and away from loved ones isn't easy. Depending on where he was he probably has some things he will need to sort through in his mind. He might have Post Traumatic Stress Disorder. Try to talk to a psychologist about it. I would suggest just googling one in your area. They chat with you over the phone for free most of the time or give you a direction to go in if you want help to understand your boyfriend.
I was in the military for 15 years and I can tell you that life is not easy. People do not understand what we go through on a day to day basis. 15 years may not seem like a lot of time to a lot of people but I can honestly say it felt like 30 years of my life gone. Despite the perks and I cannot complain about them they do not outweigh the things you have to go through when you serve. By all means I am not complaining but just giving my own personal observations of military life.
Don't be discouraged and try to hang in there. Do all you can to make the relationship work but also remember it takes two to make the relationship work just sometimes it is 50/50 and other times it is not.