Dang, that was savage. The guy was probably annoyed by something, and that boy just had to "stir up a hornet's nest". Well, a slap would have been more than enough, so the guy was quite overboard.
I also think that. Also I think the parents should teach their children some basic manner. Or at least be near them so that they can't misbehave. It's really dangerous to leave them alone like that. The world is full of all kinds of people. If the kid did that to someone even more dangerous, it would've been a catastrophe.