How do I help my 4 year old with lip picking?

I stumbled upon this site in my search for what is causing my 4 year old son to constantly pick his lip skin. Since he was an infant he has had various obsessive habits. It used to be putting his fingers in his mouth, then it was constantly making a snorting noise, then it was picking at his nails, and now we've moved on to the lip picking. He does it literally all the time & makes himself bleed doing it and doesn't even notice. It started last winter when his lips got chapped and hasn't let up since. We've tried gently letting him know when he's doing it, slapping his hand away from his mouth, cutting his nails short, talking to him about it, etc. His lips have scars on them now. I feel awful that he's always doing this. How do I help him? Any ideas would be greatly appreciated.

I know he's only 4 but having him see a child psychologist might be helpful. I started around the same age and wish I had had someone to help me before now (22). I feel resentful of my parents because they thought they could make me stop by telling me to stop, smacking my hands, and slathering my lips in carmex, but it did nothing and just made me do it more when they weren't around. They should have found me a therapist at a very early age.
Thanks for the advice. His doctor gave me the name of a child psychologist today.
Good luck to you both.
Honestly, I used to do this to myself when I was younger, and still do. Some advice I would give is introduce gum to him. It may not fix his compulsive habits, but it will put focus on him to chew the gum instead of biting his lips. It helps me, so give it a try! Sugar free gum would be the best because the flavor lasts longer and he'll be less likely to spit it out and start chewing his lips again.
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