I "cut" my wrists when I was in my early teens (actually scratched repeatedly and fairly deeply with the kilt pin from my school uniform). I did it as a way to deal with my emotional distress which was too big and too deep to hold inside, I had to express it somehow and in my house kids weren't allowed to talk back or get angry. So it was all bottled up and grew and grew until I would just have a meltdown of despair and fury. The pain I felt eased my emotional pain, it was a kind of distraction, and the injuries I caused to myself were a physical expression of what I could not say with words. My parents didn't even notice them for months, which only confirmed in my eyes that they didn't care about my suffering. When they finally noticed, they took me to a psychiatrist who pretty much said the usual stuff about superficial wounds etc. I don't believe my parents and I ever actually talked about it, it was weird. But anyway, at some point I just stopped doing it; I don't remember when or why, but I did move in with a much older boyfriend as soon as I could (16) and on to other forms of self-medication such as alcohol, which lasted until I was 19.
I don't know what to tell you, but this was my experience. I hope your son does well.