Shillelagh time

Technically he should have said "Don't worry, Mahon!" but that would probably confuse a lot of readers. Who's Mahon? Actually it's a place. It's some island that Vicount Mahon, formerly Stephen Stanhope, recently became Viscount of.

Let me explain:
The Old Earl passed away a couple weeks ago.
His eldest son was Hugh Stanhope, but no one called him that. As the heir, he held the courtesy title of Viscount Mahon. He is now Earl Stanhope. His son Stephen Stanhope is now Viscount Mahon. His brother Allan Stanhope.... is still plain Allan Stanhope.

Christian names were given but rarely used. Family that grew up together might be on a first-name basis, while even married couples might call each other Mr&Mrs Lastname. It depended on the people, of course, but these two must be close friends....