Cutting is very hard for people to understand. They percieve it as pain and unneccesary harm. It is because they care that they don't understand and want you to stop. My best friend told me why don't you rub your arm instead of cutting it after I asked her why she chewed her nails. Is there really much difference?
Try telling him you are compelled to do this and stopping is not an option at this time. You can't look to someone to stop you. That is a personal thing.