If he's hiding it from everyone it will be quite difficult to approach him. If he is defensive about cutting, as I always am, just tell him you're there for him and that you love him. If he's open to other ways of coping try suggesting anything that will keep his mind and hands very busy when he's upset. Some ideas are: cleaning (scrubbing or wiping anything works best), typing (like in a live journal, here at PC, on other forums about what's troubling him so much), writing (stories, journaling, poetry, etc.), playing sports, doing schoolwork to keep his mind busy (often I do homework, redo it just to keep my mind busy and off the cutting), dancing, talking to someone he trusts and who will support him and not tell him what he's doing is wrong or who will criticize his actions, and other things like this. For me, the worst thing anyone has ever done was try to criticize my cutting, told me to stop cutting, and yell at me or punish me for doing it. This only made the situation worse. Only he can make the decision to stop or try to refrain from doing it.

Hang in there, it's not your fault, and stay supportive of your son. He needs you very much right now, even if he does not open up to you. Just being there, letting him know that you are always there for him through thick and thin will help immensely.