Men and women think differently in terms of marriage...........if he admits that "maybe, someday" then that is almost like already asking in a mans point of view. That is setting up a commitment that he isn't ready for. The fact is he likely will be ok someday with marriage, but right now saying "yes someday" is like setting it up already.
Your allowing your feelings to get involved here and it is scaring him........the quickest way to run a man off is to jump the gun. He got scared when you brought all this up and he likely is feeling like he is on a narrow path headed right for the alter, which is very scary for some men.
At this point if you want to keep him your going to have to back down a little bit. If he brings it up again, let him know that you were just talking and that marriage is not something your interested in right now......then drop it. Do not let him see this bothers you, otherwise he is going to get the feeling that you want a ring soon.
While 2 and a half years may seem like a while, the reality is it takes a while to get to know someone and it sounds like he is mature enough to know not to jump into anything to quickly. At this point there is no reason to become upset..........he simply told you that while he cares for you that thinking about marriage is not something he is ready for.