Task: Question Answering
Input (English): Passage: The film begins with an introduction to the documentary from the boys. Nat and Alex Wolff, aged nine and six respectively, are members of the fictional band The Silver Boulders, which also consists of Thomas, David, Josh, and their manager Cooper. The band found success after a music executive signed them to his label, Who's the Man Records. The band performs their new song "Motormouth" at a concert in the Hammerstein Ballroom. After the show, the band members describe how their group started and a clip from their music video "Crazy Car" is shown.
The bandmates get along well until Thomas composes the song "Boys Rule, Girls Drool", which Nat dislikes. Nat writes a song called "Rosalina" that is about Josh's elder half-sister. Thomas and Josh ridicule Nat because the song shows his feelings for her. Moreover, Josh composes another song that Nat also dislikes, titled "I'm the God of Rock and Roll", set to the tune of "Twinkle, Twinkle, Little Star". The band has a food fight in a restaurant, prompting Thomas, David, and Josh to leave and form a new group, The Gold Boulders, managed by the scornful Mort Needleman.
After watching media reports of the band's split on television, Nat and Alex go into a state of depression. Alex begins to binge on lemon-lime soda and falls asleep, while he lies curled in the midst of aluminum cans. Nat simultaneously writes a song by the piano titled "If There Was a Place to Hide" as the band's fans gather outside his apartment, pleading for them to reunite. Despite the absence of the formers, Alex persuades a reluctant Nat to revive the band, and subsequently, they change the band's title to its original, The Naked Brothers Band. Through a line-up of auditions, Nat, Alex, and Cooper select Rosalina as their cellist and Cole Hawkins — a member of the original Naked Brothers Band — as the guitarist. 
Question: Whose younger brother left The Silver Boulders to start his own band?
Output (English): Josh.