There was recently a thread on just this subject, you might try searching for it to see all the wonderful answers, but yes teeth grinding is normal... provided he is not suffering any other symptoms. That is, as long as he is eating and does not seem to be in pain with his mouth, you should be just fine. Lots of dogs grind their teeth in their sleep as well, just like many humans do. It only becomes a problem when we begin to wear the teeth down, so as long as you keep an eye on his teeth (which, since he is a pup he will be losing soon anyway) he should be just fine.