What do I do about this other girl?

I have this boy problem…he sits right behind me and I think we have some chemistry, but then theres the problem. he has a girlfriend. he is obviously into me. like he touches me, not like a tap or a bored kinda thing, like he gives me hints. he went out with this girl -now girlfriend- for a few days, but then she got all p!ssy for no reason and dumped him. I was there for him and kinda comforted him for about 2 weeks. I started giving HIM hints after a while and things were starting to heat up, like I would touch his legs and we would whisper some dirty and sexy stuff to each other, lol. I declined his offer to go to the locker rooms, polietly!! I acted like I was being a prude or something I guess, because he backed off-unfortunately!- for about a day, but then he was back to his normal self. the next week, his ex (girlfriend of only a few days) comes crawling back saying how she was a bitch and needs him. now theyre together again! hes toned down the raunchy-ness so its at a very low minimum, but he still gives me the ‘eyes.’ ya know?? I like the guy but I have no idea what the heck I am supposed to do! I dont want to break them up, because thats not how I am, but then again, I kinda do! what do I do about this other girl? please helppp!!!