I'm not sure that Biff has an idea about the American Dream. It is clear that he has a dream for his life and it is the direct results of his life with Willy. Biff wants to get away from Willy's idea of the dream; he wants out of the office, out of selling --- he says he wants to work outdoors where a man belongs. But I don't know if this is a dream or his reaction to what he learned about the illusion that Willy lived in. It isn't so much a "dream" as the reaction to what he saw as a "nightmare."