I'm not OP, but when I used to self harm it was because I felt really ashamed to share what was going on inside. It would build up and build up but I felt like I should be able to handle it.

Try to help him find other ways to express what's going on inside, maybe like.. Writing down how he feels, or talking to someone he trusts when he feels that way. Teach him how to identify when he is feeling like he wants to hurt himself (maybe he cries when he feels that way or has bouts of angry feelings), and teach him healthier coping mechanisms.. How to express the feelings inside in a better way.

It'd be good if he could have an adult he feels comfortable to talk to, who can acknowledge how he's feeling and tell him like.. It's normal to feel sad but that if he's feeling like this often, or it's extreme, it's better to tell someone about it so they can help him. Because he's not alone and feelings like this arent good to have, and that you want to help in any way you can. Someone who can be supportive and try to talk to him about it without being negative or judgemental. Whether it's a therapist, favorite teacher, a parent. For me I ended up talking with my band director about it who I was really close with, and once I was able to express how I felt more and let it out the self harming decreased.

Sorry for the long post! Hope it's ok I replied.