Harry would see it as an opportunity for Sirius to finally have a normal life. He would support Sirius wholeheartedly. He might even ask Sirius to write down his adventures so that he can read about them, knowing that Sirius is out there living his life to the fullest after all the hardships they've been through.
He might be extremely hurt and confused. Harry has always seen Sirius as a sort of father figure, so being disowned would be like a huge blow to his emotional stability. He could become withdrawn and start to question his own self - worth.
Harry may also feel a sense of adventure. Despite the situation, he's used to facing strange and dangerous things. He might start to think about how he can use this situation to his advantage, like finding out more about his parents or the secrets of the wizarding world that Sirius might know. He could be a bit excited as well as scared. For example, he might be thinking that this could be a chance to get away from the Dursleys for good.
Sirius would probably be overjoyed at first. Harry is his godson and he cares deeply for him.
Sirius would probably be very worried and immediately try to help Harry. He might use some basic healing spells he knows.
One possibility is that Sirius, having finally been vindicated and after the chaos of the war, decides to travel the wizarding world to heal. He might visit old friends and places from his youth that he couldn't before due to his false imprisonment. He could be on a journey of self - discovery, trying to find his place in a world that has changed so much.
He might be initially embarrassed, especially if he receives it in a public place like the Great Hall.
Fanfiction about these three can take many forms. For example, there could be time - travel fics where Harry goes back in time and meets Regulus and Sirius when they were younger. This could lead to all sorts of interesting plot twists, like Harry trying to prevent Regulus from joining the Death Eaters in the first place. Or it could be a fic where Sirius survives and he and Harry explore the secrets of the Black family together, including things that Regulus left behind. There are also fics that focus on the emotional bonds between the characters, showing how the different personalities of Regulus, Sirius, and Harry interact and influence each other.
It could develop in a complex way. The spanking incident might initially create tension between them. Harry could be angry or resentful towards Sirius at first.
Another aspect of this fanfiction could be that Harry uses his Parseltongue ability to communicate with the snakes in Azkaban (if there are any). The snakes might have some information about Sirius since they can sense the emotions and secrets of the prisoners. Through this unique method, Harry gets closer to proving Sirius's innocence.
Perhaps in the fanfiction, Sirius is mad at Harry because Harry has befriended someone Sirius doesn't trust. Sirius, being protective of Harry, might see this new friendship as a threat. He could be worried that this person will lead Harry astray or put him in danger, and his anger towards Harry stems from his concern for Harry's well - being.