I don't have kids (yet) so I don't have the slightest clue.
But I'd say the most you can do is clear the air with the other kid's parents. No use trying to meddle with the boys themselves - kids will be kids, and all this stuff is just part of growing up. Coddling isn't the way to go - it's kind of humiliating.
Maybe give your kids the "why you shouldn't act like that asshole bully" talk? Or - very delicately - suggest ways they can get him back with escalating the situation with more violence. I'd go with the latter.