I don't have a preferred setting, but I like extreme scenarios. Plots where the bully and/or the mother would feed of the son's humiliation and go further down on him, or when the bully ruins mom's life just to laugh at the son. I don't like to bind my fantasy with realistic scenarios. I know some of them are fucked up and would the bully and/or the mother in deep trouble (maybe jail) in real life.
Here are some bad fantasies, read at your own risk:
1) Mom hoes around, maybe does porn, but this doesn't affect her career or social life Instead, all of the fallout of it would go on the son alone
2) Bully ruins mom's life, gets her fired from her job, ostracised from her friends, maybe get her pregnant, just to fuck with the son
3) Bully fucks son, with the blessing or encouragement of mom
4) Teacher mom has sex with the entire school to humiliate the son
5) Bully goes to fuck every woman in son's life (mom, sister, aunt, boss, cousin...etc)
6) Religious mom warns son not think about girls at all, and beats him if she suspects that he does, yet she fucks his bully like a cheap whore
7) Bully manipulates mom to make her neglect her son, physically and emotionally, maybe outright disowns him
Bully leaves mom, who can't find one to satisfy her needs and forces her son to fuck her
9) Mom fucks son, not because she loves him or because he's hot, but rather to humiliate him and remind him that he'll never get a girl
Edit: Other fucked up fantasies
10) Son has a small cock, and mom bullies him herself and works to deny him sex at all (or at least having children) so he wouldn't pass the small dick genes, even encouraging his girlfriends (or wives) to cheat on him
11) Mom makes her son go through total sex change so he'd literally be bully's bitch