assign him to the cheapest car

https://www.insurance.com/auto-insurance/auto-insurance-basics/teen-driver-which-car-to-assign.html

insurance ask for primary and secondary driver of each car.

So if you only have two cars, mom and dad are primary drivers for car 1 and 2, respectively.  They are seocnary drivers to the other cars, and the son is secondary driver for both cars.

If you have three cars, the son has to be a primary driver on one of the cars, and you should assign him to be the primary driver of the cheapest car.