Design Tips
1. Have your son draw a design on paper then cut it out and use it as a template. Maybe try using paper with a grid on it to make it easier for him.  Draw a side and top view on the paper by tracing around the block of wood. Just trace the outside of the wood block on a piece of paper.
You can use this car template (use 8.5"x14" for actual scale) if you wish.

2. Keep the car a full seven inches; it has to do with the physics of velocity and length of travel of the weights.  

3. 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.  

4. Leave a lot of wood in the rear to put in the weights.  

5. Typically the front wheels of the car will be further from the front end than the rear wheels are from the rear end of the car.
Note: If it's not obvious which end is the front of the car, the Race Starter will usually place the car on the track according to axle location instead. The back axle is closest to the end of the car. The front axle is usually furthest away from the end of the car.

6. Try not make the front of the car very pointed. It is hard to set up against the starting dowels, and when it crosses the finish line, the timer beam may not catch at the very pointed nose as it crosses the finish line, but a little back of it.

7. Use your imagination. Be creative. It seems that shape has the least to do with speed. We have had a square block of wood wrapped as a gift with a bow on top do fairly well. So a beaver driving a log or even a pickup truck is more interesting than a wedge and can be just as fast.  Other than having a sail on the top grabbing handfuls of wind, the aerodynamics of a small block of wood doesn't mean much in the thirty to forty feet they'll travel.