D.K.
My kids did this for a day or two but it didn't last longer then that. I would put him away from you when he does it and be stern. Getting negative attention will eventually sink in. Try the second you see his hands in his mouth give him a toy or distract him. It is basically a phase and you just have to keep him preoccupied as much as possible to break him of it. I think my son did it a few times and would chew on his fingers, then get them too far back there and puked. Not fun, I don't envy you but it will pass.