Do a high pitch scream (doesn't have to be real loud), so he knows he hurt you
(even if a slight nibble).  He will not bite or nibble as hard the next time. 
He does not want to hurt you.  There are ways you can learn to communicate with
him, so he understands.  I know others have listed something about putting your
hand on their head and keep the head down for a little bit, so he will know you
are the dominate one.  I've never had to do that with my bun, so others can
probably explain better.