Where Gatsby explores the concept of wealth and class from the point of view of the outsider---for no matter how much wealth Gatsby amasses, no matter how lavish his parties, no matter how desired his company, he will never be truly accepted because, as Fitzgerald hammers home in this novella just like he did in Gatsby---the rich are different than we are. Even if they lose their wealth, they are still different. The distinction goes deeper than bank account balances. It is a more existential thing, more caste than cash.
The Rich Boy is the anti-Gatsby. He is the insider. The dissolute rich boy incapable of feeling anything beyond his drive for idealized states of being---for him it is the idea of love, not love itself, or any emotion really, that drives him. Where Gatsby shows us how the wealthy damage and toy with the “common” people---The Rich Boy shows us how they damage themselves, even if they lack the ability to truly feel it.