Paul's entire childhood is based around a lie, and it's not about Santa Claus. It's about his eyes. Paul knows that someone is lying to him, but he doesn't remember the truth. This lie is a like, well, an eclipse covering up the sun. Where the sun is the truth. Get it? The lie infects everything from the family to their relationship with their community. But they can only pretend that everything is fine for so long. Eventually, the eclipse ends, the truth comes out, and we learn why Paul can't see well. But the truth is bright enough for even him to see.
It would have been better if Erik had come up with the story of Eclipse Boy, and his parents just had just gone along with it. The fact that his parents invented the story makes the whole thing worse.
Paul uses his journal as the outlet for his true feelings, but Erik and his parents don't want anyone to confide in because they're hiding something awful.