Should I be worried about him?

Probably one of the hardest emotional things to get through in life is being cheated on. You feel like crap and like no one loves you, trust me, it's a terrible feeling. I'm sure this guy had a lot on his mind when he dated that other girl for 7 months but it's obvious that he's into you. He obviously doesn't want to get hurt again and maybe by now he understands that if he wants you to stay faithful to him he needs to give you as much positive attention as possible. The guy seems nice from what you've said and since you like him then you should give him a chance. If you want to have an official relationship with him then just go for it, don't be too apprehensive about his past, we've all had bad things happen to us and if it seems like he's moved on from it then I'm sure you have almost nothing to worry about. Best of luck to you, for whatever you decide is right.