What do I do

So there's this guy I've had an on and off thing with since the beggining of highschool and I'm going into my senior year (so 3 years) we dated for a bit after that we were an on and off fling. It was better for us that way I guess (it's complicated) I love him and I always will we left on good terms. Well we quit talking because he started dating someone new about 7 months ago or so but more recently into the last 2 months he's been texting me which is fine I missed talking to him. He wants to hookup though but he has his girlfriend. I want to but that's a bitch move , and I know if i were dating him I wouldn't want to be cheated on. He insists nobody would know , but it's kind of a turn off.. You know that he's dating someone else. But he won't leave me alone ! What do I do ?