DON'T pull your hand away. That will cause the prey drive to kick in and he'll bite down harder. Just stop all activity until he takes his mouth off your hand and then praise the daylights out of him. You can offer him one of his toys, but if he just keeps grabbing your hand, hold it steady and tell him to "give" or "drop it". He'll get the idea. This is a type of ignoring behavior that has been very effective in classes that I have conducted. I've helped over 1200 families with their obedience needs and this is a very hard thing to do, but most effective if you can teach yourself not to react.