I'd guess that this hapend on Friday, last day of half-term, during wet break indoors ?
I supervise lunchtimes and can see exactly how this situation arises.
Boys are tired but excitable, there is a general air of festivity with less formal instruction and more golden time.
Pals start a fun but bouncy game, most likely a hyped-up version of a game they normally play without incident, and quickly group dynamics take over and children are splating on top of each other, without a second thought for the harm they can do en-mass or the poor children underneath.
Yes the adult must intervene instantly and put a stop to it.
Yes the adult would have to follow school protocol for the biting incident regardless of provocation.
No the adult would not be admonishing everyone provided it was a one-off, non-malicious piling-up.
A stern telling off to all and a follow through with consequence cards etc if they do it again(the piling-up), would be enough - along with standing the biter out for a bit,
For your son let him know that the incident is over and done with, that there are times when our friends get excitable and we need to remain sensible and stay out of it.