J.P.
http://www.steadyhealth.com/articles/Habit_disorders__How...
I went to the above website to read about nail biting since my cousin did this and still does sometimes as an adult. Some of the other posters said it was due to stress. But... when I think of my cousin doing it as a kid, I did not see any real stress in her. So yes... the website said there are different reasons why a person bites their nails. Your son is not alone... it stated that 44% of adolescents do this! For some, I think it's just an oral fixation. They have to have something in the mouth whether it's a toothpick, finger, etc... You may want to go to the website I typed above because I thought it was pretty informative.
J.