Ah, I see. That's a bit of a "rock and a hard place" scenario for you both.

You are right in surmising that he doesn't understand, but I think your reasoning as to why may be different. Most people who don't cut don't understand why one would do it, and don't understand how difficult it is to quit. A lot see it as a sort of cry for attention, which isn't so much the case. It's, in my experience, a very misunderstood thing.

He doesn't know what to say or do because (I believe) he is afraid. I believe he is scared of something happening to you, and in saying he will not be with you if it continues is his way of trying to get you to stop. He does care, and he does want you to stop (for noble reasons), but I think his method of trying to help is only making it worse for you. I know it must be hard on him to see you hurting and not know what to do, but he needs to be there for you. Try to make him understand that (I'd love if somehow I could address this to him).

He needs to know it's not his job to try to find a miracle fix or leverage you into a position to stop. A lot of men (myself included) take an overactive stance with things like this, when all that is needed is an open ear to speak to.