I think both Sonny and Fredo would have gladly accepted draft deferments that their father arranged for them: Sonny because he was his father's underboss at the time (and was the father of children); Fredo out of loyalty to his father's wishes.
Given Vito's money and political clout, deferments would have been easy to arrange. The novel says that even Paulie had a deferment, gift of Vito: a letter from a friendly physician attesting that he'd had "shock treatments."