I'm a Chinese writer. Chinese people always tend to refer people by their full names, so the first name last name thing shouldn't be an issue. But now I'm working on a novel with a character from the US.
Here is the scenario. Let's call the protagonist Joe, and the character from the US Adam Baker. When Joe first met Baker, they didn't have direct contact. Joe just learnt Baker's last name when his employees called him Mr. Baker. Later Baker tried to killed Joe. Until that point, Joe still didn't know Baker's first name. I, the narrator refered to him as Baker. During the struggle, they found out that it was nothing but a misunderstanding, so they made peace. Baker told Joe his first name was Adam.
They started getting closer and eventually became close friends. At some point, Baker told Joe that he should call him Adam. Is there an applicable rule for English writing? Should I continue referring Baker as Baker or should I change it to Adam? Referring a close friend of the protagonist by his last name sounds strange to me, but when I change it from his last name to his first name, it sounds too sudden, and weird.
Are there some techniques to change the name a character is referred to by without making the readers feel strange?
(I'm only talking about the way the narrator refers Baker, not the way other characters address Baker)