L.,
I'm a 34 yr old mom of 3 children under 5. I have suffered from this since I was a teen. It is a disorder, and typically a nervous habit. It is called Trichotillomania, or TTM. You can find out more about it here
http://www.trich.org/about_ttm/intro.asp.
Be loving but diligent about helping your son to stop. Make him aware as much as you can, but be supportive. If you want to talk, just email me/send me a message.
It's wonderful that you notice his behavior and that you're taking steps to help him with his problem.
Blessings,
Rolinda
Wife, Mother, Friend