Will a friends with benefits relationship with an ex-boyfriend turn into anything more?

This is a tough issue, but I don't think that your relationship with him will go anywhere. He's only there for the benefits while he searches for someone better. The implication is that the sex was the only positive aspect of your relationship.

Remove the benefits and he'll reveal his true feelings.

I know this is hard for girls to understand but we usually say exactly what is on our mind. For example, in your own description you said that he was hesitant when you asked where it was going. He doesn't want anything to change, he probably feels like making it a committed relationship ruined it. You two were great friends before, you admittedly, did not treat him well, you subsequently broke up and went back to being friends but now with the physical element. It seems pretty cut and dry, he doesn't want anything to change because you two are having fun, you're having sex, there's no attachment and no bullshit.

He is probably also concerned that if he goes back to a relationship it will turn out like it has in the past and neither of you want that. So what you need to do, if you want to be in a relationship with him, is continue this for a little while but then slowly start to prove (not say but do) that things are different and that you want to be with him. Prove that you messed up, own that, tell him how you have changed and then don't talk about it, just do it. His hesitancy proves he's learned from his mistake and is unwilling to jump back into that, and although things are different now, by you asking the "where is this going" question it may seem as though you haven't learned from the past.

Like I said, tell him why you want to be with him, own your mistakes, highlight how you will correct them, and then spend a couple of weeks proving it to him. Take him out the movies, pick up the check, bring him a case of beer, whatever, show him you want to be with him and are willing to put in some work this time.