Doctors call bikers 'mobile organ donors' for the same reason that you fear your son riding a motorbike - availability bias.
I'm quite aware of the large number of cognitive biases.
http://en.wikipedia.org/wiki/List_of_cognitive_biasesHowever I am pretty sure that the statistics of degree of injury per accident will be higher for bikers. Oh, and there's the detail that in my 20s when I was riding a bike I had two close calls that would have killed me if I'd been just slightly less lucky. One was my fault, the other 'random unforeseen event'.
I rode a bike because I couldn't afford any other means of transport. My son bought a bike for purely 'thrills' related reasons, since he could afford to buy pretty much any kind of sensible vehicle he wanted. That's what makes it stupid.
Also I've seen plenty of severe car accidents (including one of mine) where everyone involved just stood around looking sheepish after. As opposed to the several bike accidents I've seen, in which the biker was dead or close to it.
Oh, and both of the people I know who used to do dirt/trail bike riding, have permanent disability due to major joint injury.
Human body and getting stopped by immovable objects - do not mix well.
Oh, and then there's the inherent risks associated with any hospital stay. One guy I worked with had a minor bike accident, broke his leg, then in hospital got a golden staph infection in the break. Result: over a year of pain and horror. Don't know if he ever fully recovered.
If I could respectfully suggest that you would be better off persuading your son to undertake training, develop his observational skills, and above all; ride at a speed that will allow him to stop in the distance he can see to be clear, on his side of the road.
Of course I have been trying this too. In NSW there's a good motorcycle safety/survival courses run by the police. He's supposedly looking into that, and will probably do it. But still.
The 'clear stopping distance ahead' is a very basic safety rule. But there are plenty of situations where unexpected events and/or stupidity of other drivers makes that irrelevant. It's humanly impossible to predict all eventualities, so survivability when an accident does occur is still a factor. And for bikes, survivability is poor.
He's at that young age, when everything is going brilliantly, and he's maybe getting a feeling that he's invincible.
For instance while on a canyoning trip with him recently, he wanted to jump off a waterfall into a deep but narrow rock pool. Without having much idea of what the walls did out of sight under the water. Probably would have been OK. But a bad idea, that I managed to talk him out of.
Worrying though - that attitude is NOT good, in a motorcyclist.