Just tell him to phone you if he feels like doing it, often I find I can talk the anger or sadness out of me so I don't feel as bad. Just be there for him and be willing to listen.
You can't stop it, it's his choice but you can be there to help him. He'll appreciate it but if it gets worse even after you've talked to him seriously about it all then maybe confide in a councillor, they can help you together maybe
Good Luck, self-harming sucks and is hard to give up once you've done it too long, but having unconditional love is most important!
