S.L.
This is a nervous habit, I have a neighbor whose son has OCD and he does this. It really does boil down to reminders or giving him another behavior to do in place of this. Some kids pull out their eyebrows, bite their nails, etc.
My dd is a nail biter when she's anxious. We started polishing her nails and it helped to break the habit. Maybe you can give him some type of "stress ball" or object to play with when he feels anxious.