I understand your worry because he could become a sex offender. Standing over this girl watchingher like that is borderline criminal. IfI saw someone doing that around my daughter they would be finished. The thing to remember is he's a sad guy with not much going on in his life and a loser, that's why he does it. Maybe you make his life too easy? You didn't mention his age but it sounds like he needs responsibility. And most of all stuff to do, like rock climbing, surfing, stuff that will make him really buzz. The masturbation is because he's not buzzing on life. I'm not against masturbation but in between all the other things I do, I rarely have time. It's a kind of addiciton and some people will go down this path until they destruct in some way, often with sexual problems or crime. Ultimately he sounds disturbed and this is really sad, he may even have a trauma that needs professional help. If it were my child I would try an alternative therapist though, psychologists often don't understand this and plus he will be put off by that idea. Depending on his age, if you can find a good tantric or taoist therapist, that might be the best thing. Just make sure they are genuine.