We have considered Service and rank but not the AGE of said pongo. Now his real age is of course immaterial but what is important is his social age, hence the matter of calling cards.
Is he a 'new man' or is he from the days of the Raj? My colonel, some 10 years younger than me was certainly a gentleman of the latter sort. He was distinctly put out when an ex-sqn ldr, engineer, now contract manager called him "Mate". So, will he be "Sir", "Colonel" or "Bill"?
In all seriousness for a moment....
It would all depend on what is on the invitation. If it's from "Colonel and Mrs...." then kick off with calling him Colonel and her Mrs .... until invited to call him/them something different.
If it's from John and Cynthia then first names would be the order of the day.
If one is greeted with an apparation wearing a vest and tatoos proferring an open beer bottle then it would be appropriate to call him "mate" or "mukker" or somesuch.