In the novel, Henry Bowers is framed by IT for the murders that take place in Derry during the childhood-set story, and he escapes from a mental hospital decades later to go after The Losers Club while under Pennywise’s control.
Taking place 27 years after the events of the original movie, IT: Chapter 2 will see The Losers Club return to Derry, Maine for another battle against the demonic entity that haunted them as children.
We don’t know if this is how it will go down in the upcoming film adaptation, but that’s the foundation that Andy Muschietti is working with.
Moreover, the sequel is already aiming to embrace much of the weirdness and shocking nastiness that characterizes the adult half of Stephen King’s novel, so the return of Henry Bowers (and the debut of Bill’s wife) feels like a critical piece of the puzzle that will lead to a proper adaptation.