Some of the young people I've spoken too say that they cut because when the pain builds up, it feels like the only way to "let it out" the physical pain seems to grant them some emotional release.
That is exactly how it was for me. I kept it covered, wore a jacket to cover my wrists or cut my shoulders or thighs. I had one friend who knew.

Have only skimmed the replies, so not sure if anyone has suggested the school counsellor already, but it might be an idea for your son to talk to someone for his friend's sake, but also for his own - it's a heavy load to carry on your own