I've used the push the hand further into the mouth of a very obnoxious biting puppy, waiting for him to release it and praise when he does. He won't like the discomfort or losing control of the bite like that. Or, you could stand up and turn away from him every single time he does it, taking away the attention & response he likes to get.
Here is a pretty informative article about different techniques to use on youngish biting puppies under 4 mos.
Mouthing: Common Dog Aggression Problems: Training