Firstly: Good morning = Hello Have a nice day = goodbye. So you never hear them both one after the other. Good morning, Sir. = Guten Morgen. Have a nice day = Ich wünsche Ihnen einen schönen Tag.

Directly translated it would be: Guten Morgen, mein Herr, ich wünsche Ihnen einen schönen Tag. But no one would say: Mein Herr That's the problem with German, we have no word to call someone whos name we don't know. Mein Herr sounds very, but very old fashioned. so, the best thing you could say: Guten Morgen, ich wünsche Ihnen einen schönen Tag. If you know his name, so you say: Guten Morgen, Herr X, ....