What's that supposed to mean?
A formal greeting of this nature is often a sign of a good friendship. It's 'out of character' nature is most usually responded to with either:
- a similarly formal acknowledgement, where both parties are just as easily taking the piss out of each other all in good fun
or
- a comment like "get off the grass", where the respondant is challenging the greeting as though it were offered by some snooty tosser. Again, all in good fun.
Either way, it's no biggie - in fact it's a sign of confidence in communication with the other party.
However, if he had said something like "Good Evening Mr McDonald" -
then you could nuke him for this inference....