I think you have to let them have fun. Think about this. Lets say you really push the kid and he gets fast as can be. If he does not like it, he will end up quitting. Oh, he will also hate you for pushing him so hard and not letting him do what he wanted to do.
Have some fun and let the kid be 7. There are many benefits of racing even if he never gets to be great.
As a dad I think your cool to ask for others opinions.
I hope I do not sound liek an ass. This stuff bothers me. My father was a semi-professional baseball pitcher. Obviously, he wanted me to make it to that next level. One small problem....I cannot throw a baseball. No talent at all for this. I ended up hating the game.
Motorcycles became my escape. 33 years later I still ride. I still cannot throw a ball!