I have been thinking about this too as my children get older. They are quite a bit younger than your son, but I want them to enjoy cycling the way I do, without making the mistakes I did when I was younger.
Right now, my focus has been to give them nice bikes to ride and ample opportunity for fun rides. When I ride with them I do try to teach technique and how to make use of their gearing. They do a race each summer, but that is their decision.
My hope is that they want to keep riding. Years later, when they get strong enough, I would like to take them on a "B" or "C" group rides. I figure that more people they ride with the better they will become.
I have often wondered why there are not organized youth cycling teams at a school level. The equipment costs need not be that great; is it the risk of injury, or perhaps the lack of interest in the sport?