Hello NyanNyan,
(Btw nice username) I have gone through this same thing as you. however I am no longer with this person. But when My boyfriend told me he cut he said it was because no on likes him and same as you I knew it was true. He was really a sweet boy and people just judged him because he was different. I told him that when ever he feels like cutting to text me or call me. No matter I told him I would be there for him.
I feel like such a hypocrite now because I self harm.
I think you should tell him how much you love him and that nobody is more lucky to have him as a boyfriend. That he is one of a kind and no one can replace this beautiful person he is. But, no matter what during this time don't ignore him or stop talking to him. It might make it worse and he might become very suicidal. Help him get through this like I helped mine.
However, he hasn't helped me.
Be strong and he should soon try and be strong with you.
PM anytime.