I think it's best if you let him know you're there for him. It's best you provide support for him and help him to stop long term by targeting the reasons he cuts and by using some of the distractions from the link Chess reccommended.
I think you should encourage him to tell someone, but if he's not ready, having someone else know might make him worse and could ruin your friendship and he'll definately need support and friends.
Just remember he will be sensitive right now, and you shouldn't push him, be supportive and caring, let him know you want to help, and he should voluntary tell you about it, but he will probably benefit most from you just being there for him
