Why doesn't Paul tell his parents about all of the terrible things he has seen Erik do?

It seems like Paul could have avoided a lot of trouble and heartache if he had told his parents about the terrible things Erik did in Edward Bloor's novel, Tangerine, but it's a little more complicated than that.
The main reason he doesn't tell his parents is that he doesn't think they will believe him. In the opening pages of the novel, Paul is having a flashback about Erik speeding in a car and smashing mailboxes with a baseball bat. He tries to tell his parents about it, but they dismiss it.

It was "Erik." I was saying "Erik." Dad finally understood. He looked right into my eyes and asked. "What do you mean by 'Erik'? Erik what, Paul?" I stammered out, "Erik. He tried to kill me." Mom and Dad let go of my shoulders and stepped back. They looked at each other, puzzled. Then Dad raised his arm up and pointed to the right, into the dining room. There was Erik. He was sitting at the dining room table. He was doing his homework. Dad eyeballed me for a few seconds, then went out front to look for my bike. Erik called over, "There he goes. Blaming me again." Mom took me into the kitchen and gave me a glass of water. She ran a finger under the strap of my goggles and slipped them off. "Honey, you know how it is with your eyesight. You know you can't see very well." And that was that. But I can see. I can see everything. I can see things Mom and Dad can't. Or won't.

In the quote from the book above, Paul is remembering how his eyesight was damaged. It was, in fact, Erik's fault, but his parents had never told him the truth about the incident, and since it happened when he was still quite young, he has only fragments of memory that he doesn't put together completely until near the end of the novel. His mother also discredits him in this quote by talking about his poor eyesight. This causes Paul to doubt himself, which is another reason he doesn't tell his parents about the terrible things he has seen Erik do.
The third reason he doesn't tell his parents is that he is afraid of Erik. He has seen Erik commit acts of violence against others, or get one of his henchmen to do them. He has seen Erik plot and scheme against others. He sees the violence and moral turpitude of his brother that his parents are blind to. It isn't until much later in the novel, when Paul finds a group of friends and a place on the team at Tangerine Middle School that he musters enough courage to confront Erik. He also gains courage because Erik instructs his friend to fatally wound a relative of one of Paul's friends, and he sees the pain that it causes them. Between that and the continued flashbacks, Paul has finally had enough by the end of the novel, and because of this, he is instrumental in the demise of the Erik Fisher Football Dream. He also learns the truth of what Erik did to him that caused him to lose his sight.