sees that he is about to lose Willy, he does run after his, calling "Pop!" , but it is reaching out which should not be seen as a sign of a breakthrough in the relationship in any significant way.
If there is any doubt of this, it is removed in the Requiem, when Biff makes clear that, at best, he pities Willy: "He had the wrong dreams. All, all wrong. . . . He never knew who he was. . . The man didn't know who he was."
Biff might believe he knows who he is more than his father knows who he is, but all Biff really knows is that he is not his father. When he finally accepts that his father is a fake and that he himself can never live up to his father's expectations, Biff resorts to complete deprecation of himself and his desperate father
Why am I trying to become what I don't want to be? . . . Pop! I'm a dime a doz
...