M.C.
If you go to the drug store you can get some stuff called THUMB, I believe. When I worked at walgreens it was in the baby isle with all of the orajel and pacifiers. This stuff is actually to stop nail biting. You put some on his nails and then, anytime he puts his fingers in his mouth, he will get an awful bitter taste and feeling. I know it sounds mean, but that is the best suggestion that i can come up with for you.