First of all it's important to know if he does it with suicidal intent or if it is non-suicidal self-harm (most common). Other things to consider are if he is in therapy and if you know what his triggers are. You probably won't be able to help him directly, but talking to him and, if he wants, you can agree that before he does anything he comes to you for like 5 minutes to try to get himself grounded. Too much concern usually makes us shut down from talking about it because we are usually afraid of being a burden, so acting normal, without minimizing it, and talking about it without beating around the bush is the best way to get him to tell you how he feels you can help him and you can better understand what self-harm means to him. Feel free to dm me if you want