There's inherant risk and danger in freestyle Motocross and when it goes wrong it goes really wrong as your nephew found out. Humans have a way of forging ahead nonetheless usually calculating the risk, but sometimes not, and that's what's progressed us to where we are I guess.
Those people who choose to do this sport love it, it's to their core and one has to applaud that passion I think; even when they gat badly injured they come back for more.
Forging ahead .... if you survive too !
(When he was much younger and an early bike. :) )
It starts small and then, they go big. It's all that effort in between that makes them good At it and able to do those things reasonably safely, or as safely as possible.