'He's dead now'

This quote is important.
It is a present tense sentence in a retrospective novel.
It indicates how Holden, now one year older writing about his 'madman stuff', is still traumatised about his brother's death. Haunted by it.

It generates sympathy for Holden. We might understand why Holden isolates himself from a 'indifferent world'.

Think about context. Salinger saw more death than most Americans in WW2. These images of death and despair will have left an indelible mark on him, he is perhaps similarly haunted by death. We often view Salinger in conjuncture with Holden. They are both victims of trauma, clinging to lost innocence.