R.E.
I bake cakes as a hobby, and I've found the website cakecentral.com to be golden. Just sign up with a user name and password, and there are thousands of pictures of every theme imaginable on there by some very talented people. There are also forums to ask questions like this. I think you could get some great ideas there. They've been my inspiration for just about every cake I've done!
Does he have a favorite kind of car? YOu could do a sheet cake to look like a road and put a 3D car on top. I did one like that for my son's 5th birthday with Lightning McQueen. I used an 11x15 pan and the Wilton car pan. It fed a lot of people.