He is trying to play. He is still a tiny baby and does not know what is and is not acceptable. Try yelping, like a hurt puppy would, then go into another room and close the door for a few minutes. This way the pup learns that biting will only take away the attention that he wants, and he will learn to stop biting. It takes a while, but he is just being a puppy. It's up to you to teach him how to act.
Do a search in the training section fro play biting/mouthing. There are a ton of threads on this subject, because everybody (myself included) goes through it.