State insurance laws as well as insurance company guidelines differ regarding any grace period for placing insurance on a newly purchased vehicle. Due to this you will need to check with your insurance company for an exact answer regarding your son's situation.
That being said, in general many insurance companies will give a grace period anywhere from a few days up to 30 days if you are replacing one car on your policy with a different car. In this situation the new car would be covered with the same coverage as the car that it was taking the place of. So if the car being taken off the policy has only liability then the new car would only have liability during the grace period.
If instead this new car is an addition to a policy (not replacing a car) or your son will be starting up his own policy for the vehicle, since he will be the sole owner, then it would be doubtful there would be any coverage extended from your policy onto the car for any length of time. In this situation he would likely need a policy placed on his new car before driving away from the dealership.
It would be advisable for you and your son to speak with your insurance agent now about if there will be any grace period and if not what needs to be done so that your son's new car is properly insured.
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