M.J.
mine did this till they bled. I took her to the peds, they told me that she might have a vitamin deficiency. So I put her on a daily vitamin. It got only a little better. The skin wasnt so ragged. The nails grew stronger and faster but she still continued to bite. She would even bite and tear them. It was horrible. We tried everything but it was mostly habit. Now I KNOW that this wont help your son but it stopped my daughter almost in a week. Shes a pretty princess type girl, and she wanted to get nail polish for a LONG time. I was always against it cause she was a baby and that she might eat it off her nails. So after she was show interest in it I decided to buy one designed for kids and I put in on only with the talk that she can NOT bite her nails or it would come off. Sure enough she stopped after only ruining some of the polish only once or twice. She stopped at 4 now. Make sure he is not needing some extra vitamins and maybe try to find something interesting that he will like to do, otherwise at this age, as long as he is not hurting himself there is little you can do till he can explain more why he does it.