How would you describe the tone of the ending? Does the narrator express optimism? Pessimism?

The tone is a hopeful one, which would indicate that it is an optimistic one.  At the end of the story, the narrator, Sonny's brother, has an epiphany about Sonny.  He realizes that Sonny's music serves a great purpose in his life.  It is Sonny's escape, his solace, his comforter, and his friend and ally.  Without his music, Sonny most likely would have died.  The narrator had not been able to truly know how meaningful Sonny's music is to his life and what true purpose it served;when he goes to see Sonny play, however, he finally knows how important Sonny's music is to his future well being.