You need to talk to a school counselor or a parent. This problem is to big for you and you need help. Don't feel like you have to be the one to stop him, he needs serious help! The best thing you can do is just be there for him.
Send him to rehab. Therapy or counseling won't work at first. He needs to be in an environment where he will be unable to cut for a loooong time so he can detox his system. Then once he has the immediate temptation to do it eradicated from his brain everyone's help and advice may finally start to break through to him.
To be honest ( and I know this from experience ) just talk to him. He may not like it at first or at all,but he my best friend cut too.. He says he has stopped since he got his girlfriend.. ( maybe try finding him one [ if he dosent have one] or try you too dating.? Just keep talking to him, and if it gets to the point, force him to send you pictures of his arms and legs every single day... Don't tell anyone yet, that just makes it worse.