I don't want to get married yet, but I don't want to break his heart. What should I do?

Me (15) and my boyfriend (18) have been dating for around a year now. We both love each other a lot, but he wants to take the relationship farther, and I'm unsure. He's going off to Basic Training for the National Guard next month, and he gets out in the beginning of October. He told me a few weeks ago that once he gets out of basic training, he wants to propose to me. At first, I loved the idea, and was really excited, but now...I've started to reconsider. I do want to marry him...someday in the future. Right now, I just want to get my school done with, get accepted into medical school, and have everything having to do with getting into the Army sorted out, then I'll probably consider marriage. I screw up relationships a lot (not on purpose) and it's sort of hard for me to stay in committed relationships for long periods of time. The other day, he told me that his greatest fear was that when he proposed to me, I would say no. He's telling a lot of his friends that I'm the girl he wants to marry and that he's going to propose to me after BT. What do I do? I really don't want him to kneel down, and pop the question, and then just tell him "No". I know that doing that would break his heart. What should I say to him?