A former Governor from this state is now the CEO of my husband's company. There are various opinions as to how he should be addressed when spoken to: Governor, Governor Smith, Mr. Smith, or simply John. He seems to accept any of these. Which is most proper?
Since he doesn't seem to mind, as a courtesy, you could call the former governor, "Governor Smith" in person. But the proper way to address him would be to call him Mr. Smith. However, if the former governor then says, "Oh, call me John." Then, it is polite to do so, thereafter.
The proper way to introduce him would be to say, "May I introduce The Honorable John Smith, former governor of XXX." The XXX is replaced by the state name.
One title that always remains is The Honorable. "Once an honorable always an honorable" (Hickey at http://www.formsofaddress.info/USO.html).