Going Straight
There is much on the internet and books, videos and blogs about wheel and axle modifications and cautions. And since the wheels are the only thing touching the track, they are an important feature on your car.  Below are some ideas (not in any particular order of importance).

1. Place the axle in at a slight angle (5-10 degrees) where the pointed part of the nail is lower than the end. This provides three benefits: The first is that being only the inside edge of the wheel will be in contact with the track, it may help to make the car go straighter with less wobble. The second benefit is that the wheel rides to the outside of the axle and doesn't come in contact with the body. The third benefit is that since the wheel is only touching the track with the its edge, much less surface friction is produced.
I would say that this tip is for experts only. First timers will usually have trouble getting this right, so if your son doesn't have a lot of time for trial and error experimenting on axle positions, then I would recommend keeping them parallel to the ground.

2. Axles should be perpendicular and square to the body. If you have one, try to use a drill press to ensure all axles are going perpendicular to the side of the car.

3. After pressing in the axles, test the car for crooked wheels...roll it on the floor or counter top. If the wheels are on proper, the car should roll 8-10 feet in a fairly straight line. Should the car turn left or right, you need to tinker with the axle placement without removing them from the car body, until it rolls straight. Some will take a very small amount of paper and shim the front or back of the axle to true it. Patience and practice, or just roll with it.

4. One thing some more experienced racers do is make one of the front wheels a little higher than the other three. What happens is the car will only ride on three wheels, and reduce friction by 25%. Usually most of the weight will be on the rear wheels, so raise one of the fronts an 1/8" or so to get it off the ground.

5. Once you're all set, its a good idea to glue the axles in place. Careful not to get glue too close to the wheel! Nothing is worse than having the wheel fall off as you cross the finish line.