the Commander in Chief Mr. President
Mr. President is the normal title used. Sir is appropriate, as in Yes, Sir and No, Sir.
No- this is not an appropriate title for the US President. It suggests some sort of foreign royalty. John Adams may have hinted that he would like that title, so they called him "His Rotundity" behind his back.
The correct title for a former president of the United States is "former President."
No, it is not.
Barack Obama.
The Commander-in-Chief or the Chief Executive.
A current United States President no longer holds the title of a United States President when a new US President is elected.
The President of the United States.
Hail to the Chief
Commander in Chief
No US citizen may hold a title of nobility. Therefore a Vice-President, who must be a citizen of the US to hold that office, cannot accept the title of Duke of Andalusia before during or after holding that office.
Commander in Chief.