Factual error: When Frank plots the road trip he says they'll start off going up 476 to Pittston. In 1975, that road was called the Northeast Extension of the Pennsylvania turnpike. No one called it 476 until much later.
Continuity mistake: When Jimmy is sitting with Frank at the restaurant complaining about the fact that Tony Pro is late, we see Frank holding a Coca-Cola bottle with the right hand, but in the next shot the hand disappears from the table and stays down resting on the chair.
Chosen answer: First off his name was Chuckie O'Brian. Jimmy Hoffa wasn't his biological father, he was more in the nature of a foster father. Second, he probably knew what happened. What was Chuckie going to do? It was a sanctioned hit. And if he went to the police he would have ended up the same as Jimmy Hoffa. They put him in the middle of it to keep him quiet. If he goes after the people who did it, he's as good as dead. If he talks he could end up in jail with them, and the mafia will probably kill him anyway.