Whoever the policyholder is, the car has to be continuously insured, either on its own policy or on a multi-car policy. It cannot be driven or used on the policy for another car. You could suspend your son as a named driver. But you cannot legally suspend the insurance unless you take the car off the road on a SORN. As cars get older they suffer more faults like battery draindowns, so I would say the car needs to be driven a minimum 15 miles a minimum of every fortnight.