Some comprehensive car insurance policies will allow you to drive another person's car with their permission. Ultimately, you'll need to check your car insurance documents to see if your policy has a clause that will allow you to do this - if it isn't listed then you may not be covered.

It is also important to note that you may only have third party fire and theft cover for your son's car. If the car is expensive then it may make more sense for you to be added as a named driver on your son's policy.