Infinite Summer

What is the significance behind all the guys having these nicknames for their mothers, if any?

I read it as a way to show a disconnect in the how this family relates. Kind of like how Bart calls his dad Homer. The Incandezas live together and relate to each other. They don't communicate well and seem to dwell in their own interior silos. When they do talk it's as if to an acquaintance.

"Hey its Homer" said Bart. "Bart call me Father" said Homer. "Homer" said Bart. "I said Father." said Homer. "Fathomer" said Bart. "Why you little" said Homer.

It's not just nicknames for their mother, though, it's their father, too. Neither of them seem able to call him by his name--in fact, the only people who can really call James by his name are the alcoholics in AA, who tend to talk to a "Jim." These particular nicknames, particularly the Moms, sort of makes them larger-than-life, that is, it disassociates them from actually being "people." The idea of actually meeting an Avril is terrifying, the thought that that person exists, let alone is related to you. But I can understand this epic "Moms," this embodiment of such universal mothering--as in, a plurality of moms--that she somehow ceases to exist, herself; this, what Bain notes is a simulacrum's behavior, or what Wallace flat-out labels as showing anhedonic tendencies.