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

I am letting my boyfriend drive my car from Oregon to and from Canada to visit his parents while they vacation in Vancouver, B.C. from Germany for a week.

I want to make sure that his trip goes smoothly and that he has all the documentation he needs to enter and exit Canada with as little issue as possible.

Caveat:
He is a permanent US resident with a valid green card and has a valid German passport.