In reply to Sonya Mc:
An immediate reply but I'm really busy so please excuse the brevity. And I'd really recommend you buy a good puppy book - Perfect Puppy/Culture Clash or similar asap. In particular look for Gwen Bailey, Ian Dunbar or Jean Donaldson.
Basically he's supposed to bite - it's his way of learning about his teeth and how he should/shouldn't use them. He would usually do this with his mum and litter mates and they would give him feedback eg scream, tell him off, stop playing etc, until he realised what was acceptable and what wasn't.
You can't really stop them. And you don't really want to as if you did he'd never really learn to control his bite. The advice I was given is to scream and cry rather than saying no, and pretend to be hurt as well. And definitely stop the game. He'll quickly work out what's ok and what isn't.
Well it worked with mine anyway.
Good luck with it. Bet he's a darling really...