TBH, I think we're getting caught up with the reaction of the "Big Man" when actually we should be looking at the situation before.
We all take risks in life - I personally don't retaliate when my pregnant ass gets pinched by a 14 year old in his school uniform, because he's surrounded by loads of other (taller and bigger than me) mates, and I'm not going to encourage a situation I can't control.
In the same way, I no longer make wild hand gestures at dangerous and aggressive drivers, as the last time I did that, one of them got out the car and started screaming at me and banging on my window.
We all make decisions in life to behave in ways that fit in with the level of risk we're prepared to take.
This "kid" (seriously, when I read the OP I thought we were talking about a 6 year old) has looked at a situation, made the decision to behave in a certain way either without considering what risks that might entail, or weighing up the risks and deciding to go ahead anyway.
He is not responsible for the behaviour of the man who ejected him from the train, but by being a goody shit and an entitled, free-loading twat, he took the risk that someone might not like that behaviour.
Therefore, he has no-one but himself to blame for what happened next.