Here's my take:
The child who did the punching should have been brought aside and explained that violence is NOT the answer, and he should have been disciplined (Time out, whips, whatever...)
Then, after the party, I would explain to your son that everyone in the world is not a friendly as he, and some people just have "bad days", explain to him that what the other kids did was not meant to hurt him, and tell him that he will ALWAYS have a best friend to listen, play with...that being YOU.