The most important metaphor is vision (and blindness, and degrees of blindness). Paul's family acts as if he's blind, but he's not! His eyes are injured, and he has to wear those thick glasses, but Paul really can see okay. Otherwise, how could he play soccer? Metaphorically speaking, it's actually Paul's parents who are blind. They ignore (fail to see) the reality of their older son's cruelty and his crimes. All they can "see" is Erik's football talent, and they've fooled themselves into thinking that the cause of Paul's eye injury was that the young boy stared into the sun during an eclipse. Which isn't true! Paul starts to see the truth of how his eyes were injured, and it's a truth to which his parents are blind. Toward the end of the story, we find out exactly what happened: Erik violently attacked Paul, spraying paint into his eyes. Fortunately, by the end of the story, the parents' eyes are "opened" to Erik's true nature.
Speaking of the eclipse, that's another important metaphor in the story. You know how in an eclipse, one object passes in front of another, blocking out your vision of it? The same thing is going on with the two brothers in the family. Erik is the older one, the football star who gets all his parents' attention. Paul just kind of tags along. Erik eclipses Paul, and Paul has to linger unseen in Erik's shadow.
Lastly, let's consider the sinkhole. Whenever you have a sudden natural disaster in a story, it's worth taking a look at it as something more than a plot device. Yes, the sinkhole is the reason that Paul gets to change schools. But it could also be a metaphor for Paul's despair. Think about how dejected Paul felt at his first school, when his IEP prevented him from being on the soccer team. He was so sad that it was like he was being pulled into the ground, away from light and happiness. And then, the sinkhole happened. It'd be a coincidence in real life, but in a novel like this one, it's probably a metaphor.
You might look at the sinkhole, the eclipse, and the blindness, and say, "Wait, aren't these symbols and not metaphors?"
It depends on how you define these terms, and they do overlap. But in general, if you've found something that represents something else in a work of literature, and you want to know if your object or event is a symbol or a metaphor, a good test is to examine what's being represented. Does your object or event represent:
A. a specific thing going on in that particular story?
B. a general, abstract idea?
If it's A, then it's better to call your object or event a metaphor. If it's B, then your object or event is better referred to as a symbol.
In the case of the sinkhole, the eclipse, and the vision/blindness, I'll call these metaphors because they represent specific other things going on in the story, and not necessarily bigger, abstract ideas (like fairness, acceptance, discrimination, and so on).