Has anyone talked to you about anxiety? This is something to talk to his doctor about. Try above for a fix to stop the actual chewing/picking but talk to his doctor about it. Good luck
Try to replace the habit with something, maybe something else he can chew on , there are products out there one is called Mavela Stop and another Omega3 both are cream you put on well mavela may be drops .Its a tough one maybe do some research see what other patents do .