That's hard. How did you find out? Did he come out and tell you, or did you find out another way? That might be a good indication of whether he's likely to be open to support from you. There really is no right or wrong answer because so much depends on how he feels about you knowing and what kind of help he needs or is willing to accept. If you can, try to let him know that you love him and want to support him and help him to heal (remember that cutting isn't really the problem - it's just how he's dealing with things).
Here's a great website that includes information for loved ones as well as general resources and stuff to help you understand self injury:
The Coccoon: Self-Injury Pages
You can always come here, or PM me, if you have more questions. You don't have to face this alone.
Rap
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“We should always pray for help, but we should always listen for inspiration and impression to proceed in ways different from those we may have thought of.”
– John H. Groberg