I think quite possibly the reason he broke up with you is because of college. He's 2-3 years older than you. And I think he didn't want to be stuck, almost, in high school. He was going to college and he'd be dating someone in highschool (not to sound mean, honestly). And looking at it from a superficial view, I think maybe he'd be almost embarassed to be in college and have a girlfriend in her sophomore or junior year. Also from experience, around the time I graduated and started going to community college, I changed mentally A LOT! Like I looked at things differently, felt differently, thought differently. Still kind of hard to explain. And I know a lot of my friends and my Fiance did too (resulting in a 6 month relationship hiatus). Also when, going to college there are a lot more people to meet, to go out with, have a relationship with.
Also there's this thing called the rubber band theory. Im actually not even sure if its accurate or anything, but guy is close to you, does his own thing, etc. You just have to let him do his own thing. he moght go back to you, he might not, like i said though, he's most likely mentally maturing and looking and thinking in different ways than he did.
That was prbably totally confusing and if anything sounded mean, I honestly didn't mean it that way.
I also think you should ask him, like casually, why he broke up with you.