You can use this to drive certain people crazy, particularly by correctly referring to Bush's father as Ambassador Bush.
In America there is one President and one Vice President at a time, and those titles can only be applied to the person holding the office at the time. There is never more than one living person who can be properly called President. The titles does not follow a person into retirement.
"Former President" is an historical description, not a title. The moment a president leaves office he reverts to the highest title he has held short of the Presidency. (Similar to when a king abdicates and reverts to Earl of Whatever) Here are the proper titles of our last several Presidents if you were inviting them to a formal dinner (the dead ones are included only as examples):
General Eisenhower
Senator Kennedy
Senator Johnson
Representative Nixon
Representative Ford
Governor Carter
Governor Reagan
Ambassador Bush
Governor Clinton
Regarding Bush 41, in the rules of protocol Ambassador is a big deal and outranks Congressman or agency director. Ironically, GHW Bush has the same title as Carol Mosley Braun, Eleanor Roosevelt or Adlai Stevenson, all also better known for things other than ambassadorial posts. (Recall that Braun was called Ambassador Braun rather than Senator Braun in the debates)