It is perfectly normal, lucky for you its not prostitutes hee's looking at. He is a teenager and thats normal for them to masturbate.
Unless he is doing anything dodgy I'd leave well alone
Hmm. I don't think I have enough information to really give you an educated guess. It's hard to judge someone without knowing them. I'll just say, I've heard "Once a cheat always a cheat" and " Where there's smoke there's fire"