Okay, having been about everything possible for Pinewood Derby and being involved in the building about 10 cars, I can give you some tips.
Now, different packs enforce rules differently. We were usually pretty lenient, but there were cars that were made to compete in the "outlaw" class. Some packs are pretty strict and some of these tips above could disqualify your car.
According to the official rules, you are not allowed to crown your wheels, or use anything other than graphite on the axles. You must use the supplied nails and wheels
That said, aerodynamics are pretty worthless, I've seen a whole lot of brick shaped cars take a lot of prizes. It's all about getting the burrs off the nails, and aligning the wheels. Aerodynamics on a car that really attains no real speed is pretty worthless, but they can look cool. Kids like things that look cool, and it sure can't hurt.
You can get an alignment device from the Scout Shop, which can really help. They also have a wheel chuck that allows you to dress a wheel so it is flat, round, and the ridge is gone. Again, shaping is a big "no-no", so it's not worth getting disqualified over.
Remember that it's all about fun, learning, thinking, and working with your parent to create their personal "masterpiece." Don't focus on the winning, focus on the building and the thinking that went into it. You can see what others have done, then make a better car your second year. Your son will think about it all year long.
Our pack also had an "outlaw" division for Dads. You were allowed 10 ounces, solid axles, and bearings if you chose. There was a $5 entry fee as a fundraiser. I entered two cars one year and took 1st and 2nd. I took first the other two years, then retired to let others win. I just really didn't have the time to do it for a couple of years. I put some time in to a couple of those cars, but had my best time in one that took me 10 minutes, recycling the wheels and axles from the last one. If you aren't doing it, suggest it for next year. We don't have a huge pack, but we earned enough off of the entry fees to nearly pay for the track rental.