Short stories often simply reveal a character rather than offer a protagonist who changes in the story. Benjamin Button does not change because of the conflict between how old he is chronologically and how old he appears physically throughout the story. At times he benefits and at times he suffers, but he does not change or grow. Fitzgerald shows Benjamin to be as susceptible to sociocultural expectations as all the other main characters in the story.
Mr. Button, Hildegarde, and Roscoe share that susceptibility. Each sees Benjamin as somehow responsible, and each implores him to conform to the social norm. In the character of the father, we see a man whose son is not what he expected or wanted. The son, he fears, will bring shame on him. At no time does the father show any compassion or feel any empathy for his son. Hildegarde seems practical in her choice of husband. We are told she could have had her pick, so her choice of Benjamin reveals her character. She marries not for love but bases her choice on an assumption that he is a man of fifty because he looks fifty. The son is like Benjamin's father, embarrassed and ashamed because of Benjamin's appearance. Not one of the three main characters in Benjamin's life as presented by Fitzgerald ever makes an effort to know or understand Benjamin's predicament.
Where is the mother? Perhaps because our sociocultural expectation of a mother would insist she demonstrate empathy for her child's plight, Fitzgerald chooses not to develop the character of the mother. If he had presented us with a mother who refused to know and love her son, he might have lost our willingness to suspend disbelief.