Curious. Surely he doesn't have a passport from Canada still? That would just be silly. Does he vote? I hope he does but not in Canadian elections. He doesn't live in Canada and hasn't for 50 years. He has spent all of his life, essentially, in the USA. I'd say that makes him an American or perhaps a Canadian-American.
I guess my question for Mr. Shatner would be, "Are you a Canadian actor? And, if so, explain the rationale for that designation."