How long has it been since he said he needed time?
It's near impossible for anyone to tell you what's in his heart or what he's thinking. The problem is, he is showing signs of depression, but if he won't let you help him, or even allow you into his life, there's only so much you can do.
You've mentioned fighting and crying, so I'm assuming that there have been other problems aside from his distancing himself. You have to look at the state of the relationship since you've been together, ask yourself has he lied or cheated on you before because the best indication of future behavior is past behavior, and asking what's to stop him from doing this in the future is exactly right. Its very possible he could feel this way again, and close you out again.
If you can't talk to him, and get him to open up to you, all you can do is give him some time. Since it's only been a few days, give him a week without calling or contacting him and see if it helps. Eventually you're going to have to decide if you want to take the risk of waiting to see what happens, especially since he is going back into the military, you have to make a decision if you want to stay with him through that. It's highly likely he is distancing himself because he wants to lessen the pain of leaving you, not realizing how bad it's hurting you now.
I welcome your thoughts, let me know if you want to talk more.