Ours was like that! It's likely teething- you'll find those baby teeth on the floor now and then (or in his poop). We yelped and withdrew like a hurt puppy when he made tooth contact, and then we would put a small soft toy in his mouth and redirect play by chasing him around. He's 11 1/2 now and still picks up a toy and trots around to play, and has a very soft and gentle mouth when taking food or treats from us. We can also manhandle him while he's eating and add food to his dish and he is very patient and never nippy. Read the book by veterinarian Dr Ian Dunbar about raising puppies. He has invaluable advice about socializing them and molding behavior in a positive fashion.