I have two boys in scouts. One is a bear and the other is a wolf. Last year I had them trace how they wanted the car to look from the side and my dad rough cut the blocks on his bandsaw. The rest they did themselves. Sanding and painting. They glued on the weights and I placed the wheels. The cool thing our pack does is have an anything goes race for the dads or moms. I did not participate last year but may this year. The one dad cut a half moon shape along the block of wood and placed a roll of quarters in that section. Ran okay but did not win.
As far as tips for speed, as others have said, polish the axels and graphite the heck out them. Come as close to the weight without going over. I just got a mailer from scouts with some tools for the derby that allows proper wheel placement and extra wood on the side if you want to place the wheels "inside" a fender. Good luck to your son.