Here's the situation. I have a 15 year old son who is about to get his provisional license. His mother, my ex-wife, and step-father want to buy him a car. They will buy it, they will be on the title, they will be the owners. If they insure the car in their own name, is there a separate policy for my son as the driver? In other words, could I add my son to my insurance policy and he be able to drive "their" car? How does that work for my two other vehicles? And how does that work for their other two vehicles? Obviously, there will be times that he'll need to drive something other than this car that they are buying for him.

both driver and vehicle are covered on policy
your son should be covered on the car that he will drive most often and added to the family policy of his mother and step-father because they are the custodial parents and their name is on the title of the car
since he does not live with you, he will be covered if you give him permission to drive your vehicles