Assuming this in the op.
And the boy is obviously the aggressor. Agree, he could have been goaded into it but those are the consequences of being a follower, imo.
I know.
I am not siding with the guy that cold-cocked him.
What I am saying is that, just like with most things in life, it usually is not as simple as the story we all get to see.
The sad part is, these guys are the stooges and end up getting hurt for the amusment of others. That small kid probably has a broken leg and a concussion and the big kid now has a mark on his record and additional psycological trauma (he knew he went too far when he did that, that was why he immediately turned away. He was probably taught right and knew that he could have killed him doing that. It sucks when the adrennaline wears off and you are allowed to start thinking again...).
I just hope they both learned something from this. The little one learns that you have to pay for your actions, regardless of who told you to do them. The big kid, that he IS big, and that his actions can go beyond a typical schoolyard brawl from yesteryear. Regardless of motivation, if he killed that kid it would be aggravated assault and manslaughter. Not a good thing to have on your record (some places keep serious crimes on even after 18yo...).
Similar to that one father being killed by the other because they were in a shouting match at a pee-wee hockey game. Regardless of the situation, you have to know what you are capable of. Seriously injuring or killing another is never a simple situation.