Why did Biff go to Boston? What does he discover when he sees the woman? Why is it that Biff never went to summer school? Why can't he believe in his father?

Biff went to Boston to catch up with his father before his college visit. When he finds his father with another woman, he realizes that Willy is truly a fake, and does not possess the qualities he formerly believed his father held.

Biff does not attend summer school as he has given up on success - for himself and his family. His father's moral weakness reminds Biff of his own shortcomings - his proclivity to steal, for instance - and that his father's encouragement and faith in the family to succeed had no real foundation.