My T reflects things like this back on to the system. What does this part need? How does cutting help him? Every behavior is an attempt to communicate or satisfy a need. My T would ask our system to reflect on what this alter wants and needs and call on our internal compassion to help him. It all comes back to us, not T. How can we find a way to help this alter.
Often when we have a young one with such a strong need an older compassionate one steps in to help. It wasn't always that way and it took time to develop. But the focus has always been away from T and on to our system to help. T is not there for 167 hours a week. We are.

It might help to reframe it to say What is it that he needs from YOU? That's not to say that you are able to give that right now. But awareness is the first step to developing more communication. Baby steps.