Willy's feeling for Biff is complicated, as is the relationship between them. Biff has always been the favorite child, but when he catches Willy cheating on Linda, Biff loses respect for his father and no longer cares to be the favored son.
This new attitude from Biff causes Willy grief and leads to Willy's "hot and cold" feelings for his oldest son. As we see at the opening of the play when Willy and Linda discuss Biff, Willy is angry with him in some moments and in others has nothing but praise for Biff's potential.
This oscillation appears to be directly related to Willy's self-conscious sense of his own failure, as a husband and a father. Willy has hard feeling for Biff when he associates Biff with this harsh judgement of himself. (Only Biff knows about Willy's affair.)
In the same way, Willy also thinks of himself in alternating moods of praise and derision. There are significant similarities in Willy's view of himself and his view of Biff. Willy sees great potential in Biff to be a great man when he isn't blinded by distaste for his son's resentment and judgement.
In the moments of hopefulness, Willy and Biff make plans together for Biff's future. Willy's penchant for wild dreaming is transferred to Biff. Willy suggests that Biff ask for a loan from his former employer, a sporting goods store owner. For a while, Biff is convinced. Upon meeting with his old boss, Biff realizes that he had been fooling himself.
I realized what a ridiculous lie my whole life has been. We’ve been talking in a dream for fifteen years. I was a shipping clerk.
Willy's advice to Biff in this instance is based on a pretense; a dream. Just like Willy's other advice about how becoming "well liked" is the key to success in business, the idea to get a loan is part of Willy's tendency toward self-delusion.
Willy's mind is full of delusions about his own abilities and accomplishments and the abilities and accomplishments of his sons.
Ultimately, Willy's concept of Biff is very similar to his self-concept. He is alternately proud and despairing about his accomplishments and his potential.