J.B.
You have the computer at your fingertips. Go online, look up websites that actually show you pictures of the zoomed in version of what is under your finger nails. There are some really disgusting pictures I am sure. Show him what is growing with his nails. Maybe that will help. I didn't stop biting mine till I was around 18. It has to do with stress, and nerves, and is a very hard habit to break, but they do usually grow out of it when they learn how to cope differently. How many adults do you see bite thier nails? Not many. Good luck