You can't wrap him up in cotton wool forever... certainly tell him of your fears,and hopefully, he will do everything to convince you that he will be as safe and cautious as he can be. Motorcycling is convenient, fun, exciting and in better climates than sunny England, can be a most practical means of full time transport. And no, they aren't more dangerous than cars, just more vulnerable.
How does it work in the US - over here, we have to pass a Compulsory Basic Training course, then we're limited as to what power bike we can ride for 2 years (unless you do a direct access course like I did, then you can ride whatever you like). After that, you can go onto more powerful bikes. Certainly encourage him to get some good training, both on and off road - riding motorcycles isn't just about going fast, and it takes alot more skill than you'd realise to control one at a slow pace.
IMO, you need to show supportiveness WITH concerns - the minute you say 'I don't want you to have one', or similar, he'll just switch off - like most of us would!
If leathers are too hot to wear over there, there is some excellent lightweight armoured clothing available - even some that looks like cargo netting with crash protectors. A full face helmet is a must if he's gonna go fast, as are proper gloves and boots.
Other than that, you can only leave it in His hands and your son's sensibilities!
Take Care, and try not to worry too much!!