In the past, Dragon spared his time to listen to Sabo’s story. He was openly affected by Sabo’s morality and sympathized with the noble child for his refusal of his heritage.
He later rescued Sabo, who had his boat blown up by a World Noble named Jalmack, and saved him from the wreckage.
Because Sabo lost his memories of Luffy and Ace after this incident, Dragon was not aware that Sabo was a sworn brother to his son. He permitted him to join the Revolutionary Army after the boy begged them not to send him back to his parents.
After Sabo joined the revolutionaries, Dragon personally trained him, enabling him to rise through the ranks until he became the Revolutionary Army’s chief of staff. Because of Sabo regaining his memories during the aftermath of the Marineford War and the revelation of Luffy’s heritage, it can be presumed Dragon was made aware of the relationship between Sabo and his son while Sabo learned of Dragon’s relation to Luffy.
Sabo refers to Dragon as “Dragon-san” and is loyal to him and is trusted enough to lead the Revolutionary Army’s investigation of Donquixote Family’s weapons trade. Dragon also granted Sabo permission to obtain and consume Ace’s Mera Mera no Mi.
Even after regaining his memories, Sabo did not leave the revolutionaries to resume his dream of being a free-sailing pirate, showing a firm conviction to Dragon’s cause.