look up nails in the Q and A. you will get a lot of results. Also here it is officially: http://autism.wikia.com/wiki/Stimming

My granddaughters both find a rough spot on their fingers and instantly start to tear at it until it bleeds but so far they injuries although terrible to look at, our family has been monitoring this and we try to catch them as soon as they get started picking at their skin. We have taken to having a jar of natural coco butter handy and as soon as they get started, we try to smooth and massage that rough area which is temporary but effective for at least a day or so

it is a very tough problem to solve. i know of two people that similarly pick the cuticles of their nails until the nail bed is rippled and scarred. Does your son have sensory issues? i would recommend finding a good OT (occupational therapist), but you can do the following things to address a sensory issue on your own:
purchase thera-putty online, or similar.
bring him to a gym or a park often. this will help him "feel" where his body is in space.
Get him interested in sensory games that he can use his hands to play with.

when did it start? because it is sensory-related, once it becomes a habit, it will be extremely difficult to break.

Let me know if anything helps because one of the people with the similar habit has not been able to stop it for longer than a week and a half yet. if anything works for your son, she would be extremely grateful to know!

Good luck!
;DB