Be kind and nonjudgmental. Have you done any research on self harm? Do you understand why the majority of people do it? Do some research. Keywords would be "Self-Harm in Adults/Teens" (I don't know how old you guys are, so use whichever applies).

Realize that his self-harm has nothing to do with you. If you bring it up, be prepared for him to react negatively. If you are kind, he should come around after he realizes that this isn't an attack.

If you talk to him let him know that you love him no matter what and that you want to support him. Make it clear that you are not angry or upset, and that you understand that this is both a coping mechanism and an actual addiction (self-harm such as cutting releases endorphins, the body starts to crave that in times of stress).

You should never take away one coping method without replacing it with something else. Urge him to seak mental health treatment. If he will open up, ask him what kinds of feelings trigger the need to self-harm. If it's anxiety, look into the 5-4-3-2-1 method. Different triggers are going to require different types of coping mechanisms.

I can not stress to you strongly enough that they should go to a mental health professional. They are obviously suffering in some way, and the right person can help to heal that. Sometimes it takes a few tries to find the right therapist/psychologist/etc., but it's absolutely worth it.

Good luck OP, reach out if you have any questions.