At the beginning of the novel, Jim is aware of the war but not directly involved, and so he does not truly understand its terrible brutality. Ballard uses the technique of third-person limited narration, allowing the reader to experience new events just as Jim does, instead of informing about consequences and the disassociated stories of other people. Jim's detachment is partly because he does not understand the war, and later it becomes a coping mechanism to keep him sane; he cannot allow himself to fully embrace the horrors of war, so he keeps everything at a distance.
His mother's clothes were scattered across the unmade bed, and open suitcases lay on the floor. Someone had swept her hairbrushes and scent bottles from the dressing table... dozens of foot prints in the [spilled] powder, his mother's bare feet whirling within the clear images of heavy boots, like the patterns of complicated dances...
(Ballard, Empire of the Sun, Google Books)
This detached persona, coupled with a limited narrator, allows the reader to experience the horrors of war in the same order as Jim. Readers will know about many of the atrocities committed in the book, but seeing them through Jim's eyes allows increased empathy and new perspectives. Jim's personal changes are therefore partly a maturation process; he is not sure what to think, and so he tries to remain aloof, but he cannot help reacting in some ways, and the narration style reflects those reactions.