One point of flashback in "Sonny's Blues" occurs after the narrator takes leave of the young man who had been Sonny's friend. The narrator goes back in time to when his daughter has died, and he had received a letter from Sonny, from prison. It is at this point that the narrator decides to keep in touch with Sonny while he is in prison, and to maintain the relationship when Sonny is finally released.
A second flashback occurs when the narrator recalls his father and his mother, at a few different points in the past, when his mother talks about leaving for a safer neighborhood and his father says nowhere is really safe, and when the mother explains to the narrator about his father's background and asks that the narrator watch over Sonny, and when the mother dies.
A third flashback occurs when the narrator recalls how Sonny was he stayed with Isabel, the narrator's wife, before she and the narrator married. Isabel was upset because all Sonny did was play the piano all day, fail to attend school, and finally ran away and joined the Navy.
There is one brief flashback toward the end of the story, while Sonny is playing, sending the narrator's thoughts back to his deceased mother, father, and daughter. But most of that final scene, going to the bar with Sonny, is not a flashback.
I apologize for not providing page numbers, but this story is published in so many collections that page numbers would probably not help.