A natural remedy is to use some bitters on his fingers. It doesn't take very much and you can apply it with a basic kids paint brush.  It may be a little hard to find but the taste will deter him from biting.  Both of these habits, biting and picking, can be (sometimes) signs of anxiety, whether it be school, other kids, really anything.  You may really want to try and get him to open up and see if something is really bugging him?