If he normally doesn't have problems with other children picking on him, it was a game that got out of hand. There obviously needs to be better supervision on the playground during break so that these sort of things don't happen again.
That said, I'd talk to the supervisor to see what's going on and insist that there's better supervision so that this doesn't happen again. You can't ask a child to make an adult assessment of a situation and decide that they're not really in danger enough to warrant biting.
If he felt threatened enough to bite, then he felt threatened enough to bite. End of. No point in lecturing him. I'd tell him he did the right thing for these circumstances, but he had better not be biting unless he's tried everything else first.
My rule growing up was that I could finish a fight, but I sure as hell had better not start it.
I'd also make sure that these boys aren't bullying him, and if they are that the school cracks down on it. Fighting them won't solve anything, it'll just up the stakes.
Personal note, if I was being ganged up on like that, I'd bite too. Maybe he'll get a cool nickname... like Chompy. :)