Oh, high school boys.
I met my ex-fiancé when we were juniors in high school (for perspective's sake, it was three years ago.) He was smitten from the beginning, telling me all the same things that your ex told you ... that you'd have beautiful children together, that you'd get married someday, that you just didn't know it yet.
From the beginning, he said this. We 'talked' (as the kids say these days) for those two years before we graduated, but never dated until after we both graduated.
He was always aloof, in high school and afterwards. We had our first date on January 1, 2007 ... and I didn't hear from him again until well into February. He was always hard to pin down; when we were dating, I'd laugh about how he left his ex without an explanation ... assuming he'd left her for me.
But then he did the same to me, his fiancée, when we had planned to be married this August. Sometimes guys need their space; sometimes they have issues concerning settling down and convince themselves that they can and should ... when they can't and shouldn't.
So maybe that's what's going on with you. Maybe not; my ex had the Marines to blame for much of his recent incomprehensibility ... but I should have seen the signs in front of my face the whole time.
The two of you broke up once and it is probably better that you stick with that decision. Good luck!

EDIT: forgot to mention the important point. I reminded him of all the cute, silly (and slightly stalkerish) things he'd said about marriage and kids ... and I was told he'd said that to a few girls. So much for sincerity.