OK. First and foremost - DO NOT STOP the ADHD medication!!!!! Chewing on his fingernails is totally secondary to how he would feel without the medication.
On Concerta, and Vyvanse, my son starts this tick: he clears his throat every 6-7 seconds!!!!! When I talked to Dr about it, my wise doctor told me: "He is not doing it while he is sleeping, so IT IS all in his head. ALL of the ADHD medication is going to do the same thing to him. All you can do is keep on talking to him."
So this is what happened to us: (I mean, this was going on for a whole year! COuld you imagine?!!!) He really wanted this BMX bike, that was costing a small fortune. We were also getting ready for the 2-month trip to Europe. My husband, on a way to the airport told my son, that if he stops coughing the ENTIRE time we were in Europe, he will get the bike when we get back. Low and behold, he looked at us, and stopped once and forever.
Point being - it is definitely ADHD medicine that is causing it, and it looks like you tried changing them. Just try talking to him; let him know that IT IS NOT acceptable, and that the medicine is making him do it. Than promise mountains, and involve him in trying to make a "quitting plan". Keep on nagging, though :)
My son is on 15 mg of biphentin and he has been chewing his finger pads BIG TIME the past couple weeks.
Craving to chew is from lack of iron. Dramatically increase beef in diet.