Per Canadian law:
As of April 26, 2022, lawful permanent residents of the United States must show these documents for all methods of travel to Canada:
a valid passport from their country of nationality (or an equivalent acceptable travel document) and
a valid green card (or equivalent valid proof of status in the United States)
Alternatives to a passport which he probably doesn't have nor can he get:
Citizens of the U.S. who are members of the NEXUS program may present their membership card as proof of identification and as a document that denotes citizenship, when arriving by air (when coming from the U.S.), land, or marine modes.
Citizens of the U.S. who are members of the FAST program may use their cards as proof of identity when arriving by land and marine modes only.
Permanent residents of the U.S. who are members of the NEXUS or FAST programs must travel with a passport and proof of permanent residence, and may be asked to present these documents to the officer upon arrival at the border.