When the narrator goes to hear Sonny play at his club, he understands and appreciates for the first time how music has enabled Sonny to survive by offering him a way to transform his pain into art. The narrator, for the first time, is impressed with his younger brother and respects him. To signal his appreciation and approval of Sonny, the narrator buys drinks for the performers while they are on break between sets. The waitress puts a scotch and milk on Sonny's piano.
At first, the narrator does not know if Sonny understands the meaning of the drink. But right before he is about to play again, Sonny takes a sip of the drink, looks right at his brother, and nods, acknowledging the gesture and accepting the gift. The older brother has spent years worrying about, despairing over, and trying to fix his younger brother, but now, finally, as the drink acknowledges, he recognizes the value of Sonny's music as a way to deal with suffering.
A scotch is grown-up drink, showing that the narrator is accepting his younger brother as an adult. The drink also glows and trembles, representing the narrator's deep response to his brother's music.