Biff goes to Boston to tell his father that he has failed math and will not graduate from high school. Biff hopes that Willy will talk to his teacher and persuade the teacher to let him pass and gradaute. Biff, at this point, truly believes that his father is a great man.

Biff hopes that when the teacher sees this greatness, he will be convinced to allow Biff to slide. 

“[I]f he saw the kind of man you are, and you just talked to him in your way, I’m sure he’d come through for me…. He’d like you, Pop. You know the way you could talk.”

This conversation takes place at a hotel. There is a woman in the bathroom. Willy tries to keep Biff from discovering her, but fails. Willy's infidelity becomes clear to Biff, destroying the view that he once held for his father. 

Once the loyal son who took pride in his father’s self-confidence, Biff feels disgusted at the salesman’s arrogance and hollowness.

Biff's trajectory is completely changed at this point. He refuses to make up his failed course in summer school and chooses to go out west, running from the confrontation that inevitably awaits him at home.  

This lasts for seventeen years. Biff keeps his knowledge to himself, protecting his mother, but he remains deeply bothered by what he knows about his father. 

Rather than reveal Willy’s infidelity to anyone, Biff has remained silent and held a grudge against his father.

Biff struggles during the play to come to terms with his bitterness and to forgive his father's weaknesses. He makes this effort both for himself and for his mother, realizing at one point how dire his father's situation is. 

The destruction of Biff's view of Willy is central to Biff's turmoil, to his professional failure, and to the emotional challenge he faces in the play. He succeeds, finally, in learning from his father's mistakes and becoming a humble realist in ways that Willy never managed.