What Should I Do

My boyfriend and i have been together for about 4 years. Its been a rough and very off and on. But the past 2 years we have been together and its been perfect. Well i want to take our relationship to the next level. I want to get married. Hes scared that i am going to hurt him again. I joined the army and i am leaving for my training soon. Were moving im together when i get back. But i think its better to get married first. He loves me and says he wants to spend his life with me and have kids with me, but says hes to young to get married. I dont know what to do. Can someone please give me some advise...