Earl, really nice work on the cars. They came out great!
If I could suggest one thing though, I would say to add in some....how do I put this?..."wake" coming off their tires, like a wake from a boat, ya know? To show that they're in motion, and sliding around that corner like real cars would. You can build up some ridges or berms of dirt on the outsides of the tires' travel paths, and that would go a long way to making this diorama come alive. You might press in some horizontal "lines" in the track going in the direction they're moving like tire tracks, basically to show that it has been well-traveled and raced. I think this would add a little "kick" to the action in the scene!
All in all, I think he'll be thrilled with it though!