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Here are some tips for you:
Grove the axels as well. Put the axel in a drill, the drill in a vice, and use a file to cut a grove where the wheel sits (reduces some friction on the wheel).
Put as much weight as you can in the rear of the car, without the car doing wheelies. The longer that weight is on the hill the faster the car will go.
Cone the inner hub of the wheel with wet sand paper to reduce the overall contact with the body, polish with graphite afterwords.
Extend the wheelbase (if allowed). Less bumping as it goes down the track.
I have a bunch more tips so let me know how far you want to go.
Lastly, have your son work with you so he understands what the effect of your work is. Maybe I'll see you at the districts.
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