Get him to do something else with his hands. Grab a controller and play a video game? Rubik cube? PlayDough even though he's 12? Drawing?

I knew someone who broke her kid out of this just by treating it as a bad habit..every time he got the urge she put something else in his hands. Handheld games kept him from doing it in the car and in waiting rooms.

Maybe when his hair grows back he can just get a really short short fade? Will he pull it if it's that short? School will be hell for him if he has huge bald patches.