lightbrite,
All U.S. presidents retain their title for life *as a courtesy*, so all former presidents are referred to as Mr. President or President <last name>.
However, after a bit of ‘googling’ around, I see there is some difference of opinion on the matter, regarding their *official* title. This is especially relevant now, as a former president may become the spouse of a president.
Another interesting piece regarding multiple titles, is concerning the former Chief Rabbi Jakobovits zt”l. He was a Rabbi, a Knight and then a Baron and a member of the Privy Council. so his full title was: Rabbi, The Right Honourable, The Lord Jakobovits, Chief Rabbi of the United Hebrew Congregations of the Commonwealth. To quote from the London Gazette in 1981 “The QUEEN was pleased on Wednesday, 22nd July 1981, at Buckingham Palace, to confer the Honour of Knighthood upon the undermentioned:…Rabbi Sir Immanuel JAKOBOVITS…”.