Support from SO

My boyfriend and I recently got in a disagreement about what I want to do for a living. After I graduate, I plan on joining the military. He is completely against it, and refuses to stop trying to talk me out of it.
​This is something I've wanted to do for as long as I can remember, and I told him that. I've also told him I'm not going to let a boyfriend keep me from doing what I've wanted to for years.  He keeps telling me I'm going to get screwed over, and that I won't like it, and can't do it. 
​What should I do?