That's a hard one, if he doesn't like you the way you like him, I would say that it is best to leave him be. You can always try to be friends with him, maybe get him to like you by just hanging out with him. For all you know if you spend time with him he wont be able to see himself without you. This will increase the possibility of him liking you as much as you like him, and MIGHT ask you out. Just keep talking to him and see what happens. Sorry I couldn't help much :/ Good luck.
If he doesn't like u then u then u should forget about him. There's a reason why u two broke up and I think it's time to move on !! Good Luck :) and u can always get someone else that does really appreciate u