Have your son draw a design on paper then cut it out and use it as a template. I use the paper with the little squares on it to make it easier for him. Draw a side and top view on the paper by tracing around the block of wood.
Keep the car a full seven inches. It has to do with the physics of velocity and length of travel of the weights.
Use the full 2 3/4 inches (outside wheel to outside wheel) that the rules give you. This will allow the wheels to travel farther before hitting the center strip.
Leave a lot of wood in the back to put in the weights.
Use the groove closest to the end of the block of wood as the rear axle.
Note: The Race Starter will place the car on the track according to axle location. The back axle is nearest to the end of the car. The front axle is furthest away from the end of the car. This determines the direction the car will race unless the contestant clearly marks "Front" on the car.
Do not make the front of the car pointed. It is hard to set up against the starting dowels.
Use your imagination. Be creative. Shape has the least to do with winning. A beaver driving a log or even a pickup truck is more interesting than a wedge and will be just as fast. The aerodynamics of a small block of wood doesn't mean much in thirty feet.