As a nail-biter since age 4, find out WHY he is biting his nails. No accusations on your family (so please do not take it that way), but I started because of nerves (I was being abused by my stepparents).
NOTHING could stop me from biting. My mom got my nails done, I had pepper concotions put on them, I had the nail-biting gross stuff put on them, I had bandaids on them, I had gloves, heck they even stuck my fingers under a microscope to show me what grossness was beneath...nothing could stop me from biting because it was a comfort when I was nervous.
Maybe your son just likes biting his nails, but usually nail biting is a nervous habit...maybe he is being picked on at school? Or nervous about school?
Good luck!! Sorry I do not have much great advice, but I would start out with seeing if something is bothering him. Then move on to the bandaids/nail polish (there is some polish that is clear and it tastes horrid for nail biting).
Mommy of 4 non-nail biters.