Car insurance is a legal contract between you and the insurance company. Your grandson will have different characteristics regarding driving history, credit, experience, etc., so a new contract would need to be issued.
Most insurance policies do allow for permissive use. This means if you give your grandson permission to use the vehicle, he will be considered a covered driver. This is only appropriate for short-term use.
Here are the steps to follow so that your grandson is properly insured on his new car:
Once your grandson completes these steps, you can cancel your insurance policy.