Sonny is a sympathetic character, because his negative behavior can be explained by the pain and isolation he feels.
Sonny has many bad traits. He's often been the bane of his more stable, steady brother. However, these traits don't make Sonny unsympathetic. Instead, they show the results of his life experience.
When Sonny was young, his parents died. He was left alone with his brother. Though his brother tried, it was difficult for him to parent the younger boy. This led to Sonny spending more time with friends and falling into bad behaviors. He eased his pain over the loss of his parents, the lack of opportunity, and his general isolation with drugs. However, he also uses these pains and problems to fuel his music. The reader understands that Sonny isn't a bad person—he's a complete person with pain, desires, and a family who loves him.
Another reason that Sonny is sympathetic is that his brother remembers him, as a young boy, with affection. He remembers Sonny's eyes and saw them as direct, offering gentleness and privacy. This image of Sonny as a young man at the beginning of the story makes the reader more sympathetic to him, even when they read about his heroin bust.