Hey.
I'm sorry, it must feel terrible to find out you're boyfriend is having a hard time and is cutting.
I'm not really experienced in relationships, but I would say....
Have you tried asking him what's wrong? Try not to constantly pester him about it, though. Don't bring it up for a couple of days. Let him know that you respect his decision of not telling you. Give him space for a bit.
If he's hesitant to talk to you, don't push him. If he doesn't want to tell you about it, don't pressure him. Maybe he's scared that you'll leave him once you know the truth. Just generally let him know you love him (without actually bringing up this topic). No matter what. Let him know that you're going to be with him through everything. Let him know that he can trust you with a secret.
I hope I've helped in some way or the other
Good luck! I hope things go right for you and your boyfriend
PM me whenever you like. Please keep me updated on what's going on...?