score:6
According to this page from the Canadian government showing a green card is sufficient for an U.S. permanent resident to enter Canada at the land border. But for traveling by air a passport is required.
Usually, people living outside their country of citizenship can get passport issued by consular posts of their country of citizenships. However Canadian consulates in the US do not provide passport services. Instead it appears he can apply for a new passport from the US by mail -- see previous link.