He’s a wild boy raised by boars in the middle of the forest, so how did Inosuke know his name?
Inosuke was able to know his name because it was printed on his toddler-sash. He was found by a young man who lives with his grandfather, and Inosuke’s initial outfit has his full name printed on it.
He was tragically separated from his real parents at an infant age. Inosuke learns the truth about his parents during the final arc of the Demon Slayer plot.
Inosuke is able to know his name by his sash
In chapter 71.5, Inosuke’s backstory is explained. He is a boy raised by boars, and it is unknown how the mother boar was able to find him. It is likely the boar recently lost one of its own kids and substituted in Inosuke.
It is said that humans who do not learn how to talk by a certain age have problems acquiring language afterwards, the chapter explains. Yet, Inosuke is skilled with his words thanks to a young man named Takaharu. Inosuke also began wearing a boar mask at this time.
Takaharu is a young man who works all day and lives alone with his grandfather. The young man came home one day to see baby-Inosuke being fed food by his grandfather, who had mistaken Inosuke for an animal.
The man tries to keep Inosuke off the property, but the young boy would continually come back. Takaharu’s grandfather would encourage Inosuke’s presence by reading passages from “The Hundred Poets” to him.
Grandfather then lifts a sash attached to the boar-boy. The name “Hashibira Inosuke” is printed on the sash, and the man theorizes that Inosuke’s parents likely wrote the name down.
So that’s the answer. Inosuke was able to know his name because it was written on a sash by his parents. He was found as a toddler with the sash tied around him. The name was still legible by the time Inosuke made contact with the people who looked out for him.