He could be extra sensitive at going to College and something new in his life, it is a hard habit to break,once hes there maybe meeting girls and new friends he will realise it doesnt look too good. Have you asked him , he probably says" I Dunno " I have heard of nail biting so many times, it does start young and very often they get out of it , then at a nervous time ie college and the unknown looming,it rears its ugly head again. I wouldnt make a big deal of it ,its his nails that look a mess.