Well,talk to him in person tell him why he is doing that? I been thru the same trust me he can get hurt and put in a mental hospital he will stay there for days and yea he can make friends there.....ask him wats wrong?
Go over to his house and talk with him and find the details. Why is he cutting? For fun? Bullied? Tell him supporting words and tell him the cons of cutting. Cutting is really bad and it will effect his future. Good Luck