As the owner of the vehicle, you are ultimately responsible for who drives your vehicle. This is called 'vicarious liability.' To make matters worse, if your son's friend is driving your vehicle without your permission, your insurance company will not cover his claim as a result of an accident or loss. (Permission is one of the pre-requisites for an insurance company to cover a claim.) As a result, you have a couple of options. You can take the vehicle away from your son, so he will not have the ability to loan it to his friend. Or, you can title the vehicle in your son's name. This will make him responsible for the vehicle and who drives it. Also, at this point, he would be responsible for insuring it.