Also, show him this portion from Sebastian Junger's "War" and tell him to prove it wrong. It talks about a willingness to take risks in young men:
"They are killed in accidents and homicides at a rate of 106 per 100,000 per year, roughly five times the rate of young women. Statistically, it's six times as dangerous to spend a year as a young man in America than as a cop or a fireman, and vastly more dangerous than a one-year deployment at a big military base in Afghanistan. You'd have to go to a remote firebase like the KOP or Camp Blessing to find a level of risk that surpasses that of simply being an adolescent male back home."
I'm still for your son getting a bike, but you should talk to him about being safe. How would he feel about deploying to the shittiest outposts in Afghanistan?