Inform him that you require him to return it by a certain date. If he will not do so, you will remove the insurance from the vehicle and report to the DMV that the vehicle is no longer within your control and he will be unable to drive the car. I'd also ry to take pictures of the car prior that the car must be returned in the current condition or he will be responsible for any damages. I might send a written notice via a certified letter about termination of the insurance. Probably the best way - I'd do it this way if it were me but there are differing opinions.