What type of documentation is needed to drive someone else's car from the U.S. to and from Canada?

Aside from his personal documentation, he will need insurance and to be on it (call company to verify), he is definitely going to need a POA regarding him as eligible on your behalf to occupy your vehicle and to understand any implications of improper use (notary public), ensure the registration is current, ensure the smog tags are current, make sure you do preventive maintenance checks so that the car doesn’t break down half way through the trip, and check that your lights work (all of them including turn signals) as well as your windshield is free of cracks or damages. Please make sure the tires have sufficient thread. Best wishes!