Prove it
And I don’t mean that flippantly.
He was a Canadian citizen at birth by virtue of his being born in Canada to parents who were there legally.
He did not become an American citizen until later after the his mother heard back from the US consulate after all the paperwork had been processed and approved. At that point he became a dual citizen and that point was well after his birth and until then he was just a single citizen of Canada.
Had that paperwork never been filed and approved then he would have been just a Canadian citizen.