Normally when someone borrows your car, your insurance as the owner is primary while any auto insurance of the driver is secondary. So if your property damage liability coverages are exceeded then the driver's should be used as excess.

If your son's friend, the driver, does not have insurance, and your liability coverage is exceeded then you may need to check with a Colorado lawyer to find out what you could be held personally responsible for as the car's owner. The Colorado Division of Insurance may be able to give you more consumer information on your situation and the possible outcomes.