about the computer, I don’t see a problem as long as you are able to monitor what is going on there, no illegal activity of any kind. What is it he does there?
what sort of garbage on his MP4? I think this is the problem. He considers these songs as culture. I also don’t care for rap music lets call it. But for young people this music speaks to them.
I would be most concerned about substance abuse at this point. The nature of that is that, they see that a little alchohol or pot is not so bad, so then they are offered the hard stuff, which is both addicting and toxic.
I would forget about frumkeit for a while. He may likely come back when he sees that there is nothing out there but
he has to see this not be told. like others have suggested you should contact those Rabbanim and others who work with kids in this situation. It’s very likely they have learnt the nature of the beast and their advice is best.