Who is "Pop" in Bud, Not Buddy?

This is an interesting question from chapter 12 of Bud, Not Buddy.  The word “Pop” is only used once in the novel.  Lefty Lewis has just dropped Bud at the nightclub where he expects to find his father.  As Bud enters the nightclub, he overhears a story being told by one of the band members and recognizes the story as a complete exaggeration.  The sentence that has the word “Pop” in it is as follows:

And to make a long story longer I go out and flick this halfway stiff right jab clean at Pop’s head.

The man speaking (who we eventually learn is Herman E. Calloway) is using the term “Pop” to describe a man named “Jordan ‘Snaggletooth’ MacNevin.”  MacNevin is a fighter from Chicago.  When Calloway was young, his manager set up the fight for Calloway.  The story, and the sentence that references the word “Pop,” is about how Herman E. Calloway wins this fight.  In this instance, the word “Pop’s” is a derogatory term for the fighter that implies he seems older than his age and unable to fight well.  Herman E. Calloway easily wins the fight by using a punch called a “stiff right jab” at MacNevin’s head.