Who does Paul think is responsible for the incident he remembers?

Paul is quite certain that his older brother, Erik, is responsible for the incident that Paul remembers.

If you look toward the end of the chapter called "Friday, December 1," near the very end of the novel, you'll see where Paul recalls the entire incident that damaged his eyes when he was five years old, the incident that he had struggled to recall correctly. All his life, his parents had told Paul that his eyes were damaged when he looked directly at the sun during an eclipse. But Paul has a feeling that this is incorrect, and that his older brother Erik is somehow responsible for the injury.

When Paul finally recalls the traumatic event with clarity, here's what he remembers: Erik, along with his "goon" (his dumb, violent friend named Vincent Castor), had attacked the young Paul. These kids were angry with Paul for tattling on Vincent, who had been spray-painting graffiti. Erik held Paul's eyes open while Vincent sprayed the white paint directly into Paul's eyes. Paul also recalls screaming and rolling around immediately afterward, then being driven to the hospital by his mother.

As you can see from Paul's recollection in that chapter, it was mainly Erik (plus Erik's vicious buddy Vincent) who should be directly blamed and held responsible for Paul's eye injury.