Maybe this is where all the 51st state talk is coming from. If Canada joins the USA then he'll suddenly be eligible to officially become president.

Don't you have to be born in the US to become president there?

No. Just be a natural born citizen and to have been a resident for the past 14 years. Natural born citizens are those with birthright citizenship, and people who are the children of citizens have birthright citizenship, no matter where they are born.

Elon is a heridatary citizen of Canada, and so would be a natural born citizen of the US if we joined the union.