Should my son add me to his insurance policy as a named driver?

Is it permissible for me to buy a car for my son and let him pay the insurance with me as a named driver?

How do we switch the insurance on three cars around without incurring costs?

Three members of my family are all with different insurers on three different cars. We are planning to switch the vehicles around - how do we do so without incurring too many costs in the process?