I'm a bit split on this. If I were you I wouldn't want to use fear to prevent my son from doing something. My parents tried it with me. My mom would have gone crazy if I had told her I want to get a motorcycle. So I didn't tell her. I went out, got a bike, got gear and showed up one day at their door step. She didn't talk to me for a week.
I think part of the reason why I got it was to prove to myself that their fears are not my own. Keep in mind, this is at 28.
That said, 18 year olds are reckless and think they're invincible. But like anything (sex, alcohol), promoting safe practices is a lot better than trying to force abstinence. Talk to your seen, dig into his reasons why he wants one. Check out the new rider thread and good luck to both of you!