Say that's great, leave your girlfriend then we'll talk? He cant help his feelings for you. I dont blame him for that. However, he's not supposed to act on his feelings if he's got a girlfriend. Either he works it out with her or he leaves her THEN goes out with you. Right now, you dont do anything but stay away from him. If he's the type of guy who you should be dating, he'll leave his girlfriend.
You realize there is a huge red flag...he has a girlfriend and hes betraying her. If hes doing it to her what says if hes in a relationship with you next he wont do the same to you. Commen sense can save you from this mistake.
Tell him to come back when he's unattached.
Move on, girl.