Can the insurance company deny a claim after an injury investigation?

There may be exceptions, but under most policies you are covered as long as your son had your permission to drive the car. At one time there was an amendment on my policy stating that my car was NOT covered if my daughter was driving (after several tickets and three at-fault accidents), but that would not be normal. If your company is investigating, that probably means they are trying to determine if there is any validity to the other party's claim. They would not have to look any further than the policy to determine that your coverage did not include the injury.