"Fredo turned out to be nothing more than an innkeeper and ladies' man, the idiom for ladies' man untranslatable but connoting a greedy infant always at its mother's nipple--in short, unmanly."

This points out how Americans and Sicilians define a real man, as Puzo sees it. In America, a real man is successful with women. In Sicily, a real man takes care of his family. (Refer to Vito's statement to Johnny Fontaine, but really directed at Sonny.)