I never won anything more than den-level, but my brother won the pack trophy for 3 years, we even had an old two-lane track my grandfater made for his dad so he (my dad) could test his cars when he was a cub scout.
Speaking from my experiences (building our cars was a family thing, all three guys in the family worked on them):
Do NOT use the pre cut axle slots, drill your own, those slots arent accurate enough for the axles, use a drill press or milling machine. Three wheels on the table is gospel.
Chuck each wheel up in a drill and sand them so that its slightly rounded in profile. That way, only a small line of the wheel touches the track, instead of the whole surface.
To insure your son's car is within weight, have him mill an area about 1/4" deep in the bottom of the car, covering about 1/2 of the bottom surface area. Take fishing weights and squish them flat in a vice, and then *hot glue* the squished weights and lots of BBs in the area. If he is over weight, he can use his pocket knife to pop out either the BBs or the fishing weights to lower it. I always pretested my cars before the race, most I ever had to pop out was 10 BBs. Those kids that drill, put in lead weights, and fill with wood putty are up the creek without a paddle when they find out they are a full ounce over the limit.
I found that car-shaped derby cars work best. Mine was like: 1/2" tall from the hood to the windshield area (about 2" long), then about .8" tall in the passenger area (1.5 inches long, curving downward on both sides to meet the sides of the body, and also curved down on the front and back of this area, from the center of the compartment, if the curvature was function f all would have f' < 0 and f'' < 0), then a few more inches and a spoiler thats the same height as the body compartment, if not a little lower (again, if curve f and starting from the car body: f'>0, f''>0).
Dry powdered graphite is the best lube out there, but use it VERY sparingly, a little goes a long way.
Kharn