Honestly, the "my good man" was just how Alastor talked in general. Part and parcel with the Transatlantic accent he used, most likely.

Alastor released the man's hand. "Dreadfully sorry, but should I be calling you 'Mr. Mikotoba' or 'Mr. Yujin'? I know some people tend to introduce themselves surname first as opposed to the American tradition of given name first. It's a bit of a change to get used to but it's hardly polite to make assumptions."