He specifically only really has nicknames for the male characters in my head canon.
Henry Stein
"Hen"
Daniel Lewek
"Buddy" obviously but occasionally he'll call him "Bud" or "Danny"
Samuel Lawrence
"Sammy" obviously but also sometimes "Sam"
Johnathon (he doesn't have a canon last name .-.)
"Johnny" or "John" cause duh
Jack Fain
"Jacky"
Walter Franks
"Wally" of course but also "Walt"
Thomas Connor
"Tommy" or "Tom"
Norman Polk
"Norm" or "Normie"
Grant Cohen
"Accountant" though that's not necessarily affectionate, and "Gee" (pronounced "Jee"), Grant hates it with a passion
Shawn Flynn
"Shamrock" but it's very rare
Bertrum Piedmont
"Bertie" obviously but also "Bert"
Bonus:
People will sometimes call Joey "Jojo" as either an affectionate name or just a means to get back at him
