Let’s see here…
Your kid is a bully. He starts to believe his own propaganda and takes on a kid twice his size, and gets body slammed for his error in tactical judgment. The event has gone viral on the Intertubz because people are getting sick and tired of the kind of crap your kid perpetrates and are tickled to death to see someone fight back successfully. And you want an apology?
I think I see how your kid got to be the way he is. Up yours, Mom.
Sounds like it is time for one of those apologies – y'know, "I am sorry your son is such an idiot that he tried to pick on my boy, who is coincidentally twice the size of yours. I am doubly sorry that your son's abusive actions forced mine to defend himself (I mean, really, my boy could have sprained something!), and I am even more sorry that your son could not keep his violent behavior in check.
Oh, by the way, piss off."
Or something along those lines…