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.