Boxing was a way for him to 'prove his manhood'. [=to prove that he was a strong/brave man]
In his early 'manhood' [=when he was a young man] he decided to become a doctor.
He grew from boyhood to 'manhood' in a small southern town.
(..) because aside from the fact he has the same frailties as all human belings. he's the only father I have. Therefore, he is my model of manhood, and my estimation of him will govern the prospects of my adult relationships. So I hope you bear in mind that any knock at him is a knock at me, (..)1989 TvShow: The Simpsons Title: Simpsons Roasting on an Open Fire Season: 1 Episode 1