How can I drop my 18 year old son from auto insurance policy?

Yes, You can drop anyone from coverage on your Auto insurance policy regardless of age or relationship. Contact your Insurer or your Insurance Agent and have him removed from your policy. You may also need to sign a form 515A to have the driver excluded by name. Many Insurers will "Falsely" claim that you cannot remove a child or other household residents from your policy but just be persistent. If your insurer refuses to remove or exclude the driver then they are likely trying to gouge you for additional premium and you should contact another insurer.