Well, I would say just forget about him. And if you didn't want him swimming in the neighbor's pool with no shirt it's not really your problem. You gotta let him make his own decisions too. And if you didn't want him to tell anyone you were dating then maybe you shouldn't have dated him in the first place if you couldn't trust him. Now, if you're in middle-school or younger, my advice would be to just not date and forget about it until you get older. But if you're older maybe confront him about you feel. But since he has a girlfriend definitely do not try to make a move about it. Hopefully you guys can be friends and get it sorted out