As a parent... I too would be worried.
I am a Harley rider of many years. I have seen what you fear. I endorse good riding training... but let's face it: Young people have little fear and that frequently leads to disaster.
With this in mind, I would insist that my child view some pictures of some horrific disasters involving motorcycles. (I will not post this link on this thread. It's too graphic. I will send the link to anyone that privately messages me). I would intentionally strike the fear of god in him with these pictures. This is the only thing that I can think of that would work - at least on my kids! (which I did - and it had the effect that I wished. Their unbridled enthusiasm changed to serious respect for the vehicle. They slowed right down.)
I will say this though: DON'T (I mean this!!) ... DON'T look at the pictures yourself. For obvious reasons, you will have worse fears than you have now. It's hard to view.
This idea may not be appropriate for your child, nor the way you wish to communicate with him. I respect that. I will leave this to your discretion.