Hi Lola,
I was a nail biter, fingers and toes, all my life (33 years of biting), as long as I can remember, and only stopped earlier this year when I had morning sickness, fancy that! Now, I still nibble occasionally. My parents divorced when I was about 2, I'm sure that is what kicked off my nailbiting.
I will second Clydesdale advice of not making a fuss about it in a negative way. I reaslised later in life that I would bite when I'm bored, when I needed comfort/felt uncomfortable in a situation (I would bite, even though it was painful and bleeding, but just could not stop, like a compulsion), or when there is an imperfection, such as a bit of loose skin around the cuticle or rough nail. I would bite to 'perfect' it, wich would (of course) not work, and then bite some more to fix it,etc. I know this sounds sick and ridiculous, but that is really the way it was for me. Maybe it might be worth observing your son and seeing when he bites/what kicks it off. Maybe make a big fuss about buying him a nice nail grooming kit. A few years ago I succesfully stopped after substituting the impulse for something else (adapted from a tip for dieting that I read somewhwere). I carried a tube of handcream with me always, and whenever I had the urge to bite, I would apply handcream and rub my nails instead of biting, to a rhythm, like 5 circular motions to each nail, or something like that. Substituting the impulse in a rhythmical way for something else. As soon a little white bits appear, LOTS of praise or rewards (read expensive new coat for me :-))
Writing all of this down and reading it back makes me sound like a nutjob! But I hated my hands, always hid them, am in a professional position and biting your nails in meetings etc. is not on! so if you can help him to sort this out now, he will really thank you later!
Hope this helps and good luck!