If you yelp loudly "OW!" the puppy will learn when he nips it hurts. You should also leave the room for a few mins and ignore the puppy after she nips. I know some people think that holding it down can be bad for the puppy, but some people advocate it. Puppies in general are very jumpy and mouthy, so I wouldn't chalk it up to anything else
How is your son dealing with it?