In “the scarlet ibis,” why does the narrator rename his little brother doodle?

The narrator, named his little brother Doodle when he was still 3 months old. The narrator named his little brother Doodle because when he crawled, he crawled backward and it made him look like a doodlebug. And for him (the narrator), naming his little brother Doodle was the nicest thing he did for him. Hope this answer helps.