If your 17 year old has moved out of the house you can inform your insurance company and request for him to be taken off of your policy. The insurance company might request a copy of your son's own policy as you noted or other proof that he has changed his address of residency.
The guidelines of how to take a driver off of a policy is up to each individual company. We know of instances where it has been difficult for a person to prove that a driver is no longer in the household. The reason each insurance carrier is so picky about it is because they may still be liable for damages if the driver is in fact still living in the house.
Most company procedures require that the driver being removed send in proof of residency outside of your house. This typically can be in the form of an electric or gas bill, voter registration, vehicle registration college registration or other type of proof.
You will need to find out what proof is required by your insurance company, if you are unable to obtain that proof you may in fact have a hard time removing him from your insurance policy.
If this situation makes you start looking for a new insurance carrier you can start here by getting quotes from an Auto Insurance Specialist.