Wasn’t the case in 1970:
Declaration of Renunciation (was taken at the same time as oath from January 1, 1947 to April 2, 1973)
I hereby renounce all allegiance and fidelity to any foreign sovereign or state of whom or which I may at this time be a subject or citizen.
Oh good - then you should have no problem finding the document showing his mother made such a declaration if she became a Canadian citizen. If you can't provide such documentation, then any claim that she did so is wild speculation by another round of birthers.