Many have been commenting on the perp, but for a moment I would like to comment on the victim.
No judgement here on the kid who got laid out, but it is time for us to teach our sons and daughters to be conscious of their surroundings. Having lived in Latin America for three years, I learned the hard way that you must always be aware of what is going around you, who is moving up behind you, and try your best to avoid placing yourself in a position where you have blind spots.
I practice this all the time at WalMart, at fast food stores, walking urban streets, etc. I am always conscious of exits and also aware of the people around me and their behavior. It is impossible to be omniscient, but intense practice does help in reading your environment and makes it tough for the thug with brass knuckles to sucker punch you from behind. Try the experiment the next time you are in a big store. Relax and let your awareness spread like a blanket around you. No jerky movements, but slow casual glances around you, ready to ease into an aisle if somebody is walking up fast behind you or in front of you. It's an intriguing exercise.
I'm going with friends to a fireworks display on Monday and when I do, I will be carrying a Cold Steel 37" Irish Blackthorn Shillelagh Walking Stick. In my 60s, I look like I might need it, but I've practiced for hours with the thing. I'm realistic; a dedicated thug or gang could take me out, but at least somebody is going to be missing a mouthful of teeth before I go down.