Tom,
I ran several Pinewood Derby races during my tenure as Cubmaster. Here's a
link to the last set of rules that we raced with.
http://www.cubpacks.org/ks/cs/899/PW_Rules_2004.pdf. You need to contact
the pack's leadership and see if they have their own "official" rules. There
are some links to good websites at the bottom of the rules that have some
designs for your son to look at. Above all, have fun at it, and let your son
do as much as possible. We also had an Outlaw race that was open to all
comers, mainly to try to give the dads an outlet for their own ambitions.
Here's a good set of general things to remember:
http://www.cubpacks.org/ks/cs/899/10...od%20Derby.htm.
And on the lighter side: http://www.cubpacks.org/ks/cs/899/signspwd.htm.

If you have any questions just holler, and I'll see if I can help you out.

Murray DeVore