Legitimately part of his issues could be the head trauma he received as a child. The prefrontal cortex seems to be responsible for personality, decision making, and social behavior. Even if the skull injury was to the back of the head the brain would have bounced forward after impact and could have caused injury there.
Boop. This. Some form of personality/behavioral change is fairly common after TBI, with executive functioning and emotional regulation issues being among the easiest to spot. I think the last time we see him with iris/pupil distinction is
as he's falling. Once he turns up in the hospital a week later, he's not only dead-eyed but has a flat affect instead of his previous cheery little kid bearing.
Then again, in the author's mind, maybe the emotional betrayal alone justifies the change.