How Do I Tell Him?

I really don't know how to start this off, but here it goes. My boyfriend (Chris) and I have been together for almost 2 years now. I love him so much and although I'm only 17, we have talked about marriage before. Before he contemplated proposing before he deployed or after he came home. That obviously didn't happen because he's deployed right now. But anyways, a couple of days ago, he Facebook messaged me and said something along the lines of, "Any chance you wanna get married soon?" My first reaction was YES! but...... I'm also just a senior in high school. So my obvious first question was how soon are we talking about? He said within 2 years of him coming back. So that means by the time I'm 20 if I wait for the full 2 years. I know when guys are in the army, they think about getting married and having a family, and I want to have all of those things with him, but I almost feel rushed. He has a plan for when he wants to get married and he really wants kids, but he needs to realize that I'm 3 years younger than him and that he needs to give me some more time to grow up and figure out what I want to do. I mean, yes I know I'm young but I know that I want to be with him. But I also have my own dream. He'll be coming home and he'll want his dream of being a dad to come true. But I want to go to college, and become a vet. That will take at least 8 years. Then I need to get my career started. I love him so much, but I don't want my dream to die. I definitely want kids but this is too soon. How do I make him understand that I want to be with him forever but that he needs to give my life some time to take off? Any advice is greatly appreciated.