What to think of boyfriend's sudden change in behavior?

Having been in the military I can tell you from experience that it's a life changing event. If he's been overseas like I have been, he won't be the same person he was before he left. It sounds like he's wanting a very big commitment from you as he's gotten from the Military. It seems like he's trying to transition parts of his Army life to his personal life. If your comfortable with the idea and you love him I say go for it! Most men want to get married after boot camp because they're scared if they leave, then the woman will fool around on them and they just want someone they love to come back to. Hope this helps!

He is ready for the next step. He is leaving and wants you to be with him.

Sounds to me like your boyfriend did a lot of growing up during training or his tour. He probably wants to be responsible, has become disciplined, and he's made that transition from boy to man. He wants to quit messing around and settle down. I think it's a good thing! It's very rare and you're very lucky!