Help my brother to be happy and stop self harming

About a month ago I found put my brother has been cutting himself because of some mean people at his school who are being horrible. I was really shocked and horrified I guess, just because I never suspected anything. Normally he’s a really happy and optimistic person so I just couldn’t understand what made him want to hurt himself. Anyway, since then mum and dad know about it, but I guess, in response to us all finding out, he’s started cutting himself further up his arm so it’s under his t-shirt sleeve and not as easy to notice. When I saw I just didn’t know what to say or do, and am now worried that he’ll do the same but somewhere else on his body. Now every time he’s late home, or is shut away in his room I’m worried that he’s hurting himself and I’m really scared that when he goes to uni this summer there’ll be no one to keep an eye on him and talk to him and it will just get worse and worse.