Sorry to interrupt but Van Helsing is correct:
MISS MANNERS
My son called me from the airport and told me that when he was boarding the plane, he noticed two men with wires coming out of their ears scrutinizing each passenger. While settling into his seat he heard the people behind him say, "Yes, it's him, all right, but he looks older."
Later, he was surprised to see Jimmy Carter coming down the aisle shaking hands with all the passengers. When he reached him, my son stood up and said, "It's an honor, Mr. President," as they shook hands.
He then wondered what one calls an ex-president. Surely not, "Mr. Ex-President."
Since it had never occurred to me to teach him the proper way to address an ex-president (I didn't know myself), I promptly went to my two (perhaps outdated) etiquette books, and both of them agreed that an ex-president is addressed as "Mr." as in "Mr. Carter." One said that very close friends or former staff sometimes use "Mr. President." Now, I am told that when he was recently interviewed on television, he was called "President Carter."
I have been told, also, that our ex-president likes to travel and pops up here and there, surprising people. Please let me know what Miss Manners thinks is the correct way to address a former president.
GENTLE READER
What do you mean, "what Miss Manners thinks"?
Who should be president is a matter on which the citizens are supposed to exercise their opinions. Etiquette is not.
What Miss Manners can give you is the correct information. But it comes with a warning that most people not only don't know or believe it, but turn indignant with the notion that it is disrespectful.
This is because we suffer from title inflation. Our Founding Fathers, including the ones to whom this question applied, established American protocol to be simple and unpretentious - and thus antithetical to the modern taste.
Nevertheless, the rule is that titles pertaining to an office that only one person occupies at a time are not used after retirement. A former president can use a previously held, non-unique title, as the first one did by reverting to Gen. Washington in retirement, or the plain citizen's title of "Mr." The third president preferred to be known as Mr. Jefferson rather than Gov. Jefferson. Thus, the gentleman your son met would be addressed as Gov. Carter or Mr. Carter.