Your employee's insurance policy in general covers him when he drives other vehicles but as excess or secondary to the owner's car insurance. If his insurance would extend as secondary insurance to a company car or work vehicle would depend upon the terms of his policy and the guidelines of his insurance carrier.

The SR-22 which is attached to his insurance is not a type of insurance but a certificate or form that the state is requiring him to have filed to prove that he is maintaining the insurance on him that the state has mandated. Typically this insurance is the state's required liability auto insurance.

Your employee can find out from his insurance agent to see if his personal policy would cover the company vehicle but keep in mind that the company's high risk policy will be primary and needs to remain on the company car so that it will be properly covered.