. . . and "citizen" implies current citizenship, not revoked/abandoned citizenship via marriage of the mother of a minor child to a foreign national and subsequent adoption of foreign citizenship.And the standard of proof for that is within the purview of every state's legislature. As SCOTUS noted in the Bush v. Gore decision, each state legislature has the authority to select the state's presidential electors directly without even conducting and election, at its discretion.
His father was NOT a US citizen at the time of his birth.
According to the US State Department, citizenship is not revoked or abandoned by marriage, adoption, or adding a second citizenship (unless the second citizenship is with the specific intent of renouncing US citizenship.)