I feel any judge or jury would see the obese child as defending himself. From what the video shows, he neither threw the first punch and it was clear that first punch was not going to be the end of the event. To top it off, the obese child did what he had to and did not continue beating the small child when was down.
If that obese child was my son, I would not congratulate him but I would make it clear he acted proper. Heck, I'm not the type to sue, but I would in this case for suspending him.