Flexible car insurance to add/remove driver frequently

[TUS373]

Getting a new small car, an EV very soon. My son is 19 and has his own car and policy.

The EV is £350 to insure fully comp for my wife and I. Adding in my son takes it to around £1700, so an extra £1350. The thing is, he's at uni most of the time. It would be the holiday weeks like Christmas and Easter when he could actually use it. Its a waste to pay £1350 extra for years round cover when 80-90% of time he is not here.

Is there any form of policy that is cost effective and makes it simple to add him and remove him from cover ad hoc?

Thanks.