Sorry to hear. It is people with his attitude who get hurt, regardless what type of bike they are on. It doesn't matter about size, it matters about the maturity of the rider.

Hopefully he recovers well. Maybe he'll learn from it. Took my uncle to lose his leg to finally learn.

 

I've seen injuries like that when people get thrown off the bike and impact the clip-ons or even the windscreen.

The impact could easily fracture a rib, which them moves and causes liver/lung injuries.