He took a few hits, got pissed, and took action. If it were my son (the big kid), I would have told him he did nothing wrong. He defended himself. He didn't pile on afterwards to beat the little kid up, etc. He ended the altercation quickly and effectively. This is clearly a case of the little kid picking on the big kid.
I was in 3rd or 4th grade and while not big and fat like that kid, I was definitely bigger than most kids in my class. I remember being picked on and telling my mom. I must have cried for 15 mins about how I was being picked on. She listened and said "if he does it again, you punch him right in the nose." The next day or a few days later, I told the other kid that if he did it again, I was going to punch him in the nose and I wouldn't get in trouble (silly words, but yeah, I was a kid!). Anyway, the principle got involved and my mom was on my side. If the school isn't going to stop it, then I was within my right to defend myself.
Flash forward to when these guys are adults.. If that same thing were to happen, there would be no charges against the big guy. He didn't swing or cause the problem, just defended.
Again, good for him. It's a shame he was suspended.