Do you think the father or the son was in the wrong?

We were walking through a department store today and the man in front of us was sneaking up behind his son (I'm guessing son) and yanked his pants down and they fell to the floor. The kid was maybe 10 or 12 years old I would say and apparently mortified. He pulled them up and and turned around quickly and punched the guy right in the stomach. The guy had his hand over his stomach and then yelled at the kid about how it wasn't right. I'd say turn-about is fair play. If you are going to do something as stupid as that to your kid in a public place where all can see, then don't expect a logical reaction! He got what he deserved.