Tiz - sorry to hear about the fight, I would be proud of my ds for standing up for another child, but of course worried about his safety!
To be perfectly honest, my ds has been on both sides of that fence - standing in between a 'bully' and a smaller kid he was defending, and getting too carried away with a shoving game among several boys and girls. Hopefully a good frank discussion among parents and faculty will help guide what next steps are needed~