T.L.
im 19 and still bite my nails lol. i hate it, but i cant stop either. my parents tried everything it didnt work. however my mom told my little sister when she was about two or so, that there were "buggies" under her nails. so everytime she had her fingers in her mouth, she was eating bugs. it worked with her, however your son being a boy, he may enjoy that lol idk. they do make some stuff you can put on their fingers/hands that tastes either sour or spicy, that is supposed to work. kinda like the stuff you put on your furniture, when you have puppies that taste horrible, so they dont chew on it. i just thought i would throw that out there. i hope you figure something out, because not only can it make him sick, it is a habit that is very hard to break, even when your older...trust me i know!