How to ask a family member to stop picking

I am really struggling because my dad picks his head constantly. Whenever anyone asks, and I mean gently asks, he gets really angry. I know he is embarrassed but it makes me sick to my stomach to watch him. My mother even started sitting in another room at home because it is so hard to watch. Now they are visiting for Christmas and I can't even look at him while he is talking to me because he is at his head ALL THE TIME. I can't believe that he doesn't realize that most of our conversations are had with my face turned away from him. Last night my mom asked him privately " could you try to do that less, or do it in private if you need to" I noticed that he still could not resist for the rest of the night but would catch himself. Today though he is back at it in full force. I have struggled with the same thing in a less severe way, but my husband really helped me to overcome it. So I can relate to what he is going through. The difference is that I wanted to kick the habit, I didn't want anyone to feel around me the way I feel around my dad. I just want it to stop. I don't want blood and scabs on my furniture, I don't want him holding my baby ( or touching anything else in the house for that matter) after he has been picking and eating his scabs...(he rarely washes his hands) I am desperate for help because he becomes so angry when anyone mentions anything but at the same time I don't think I should have to tolerate/enable it because I might hurt his feelings. Does anyone have advice for how to help a family member stop picking when they are resistant? And if they refuse to make an effort, what sort of boundaries do I set without alienating him? I don't know if people who pick know how harmful it can be to relationships...can anyone help? Thanks.