Bernard's dynamic character is on display in his father's office for Willy to observe. Willy would hardly call Bernard "anemic" at this juncture.
Bernard has become self-confident through education and experience. Perhaps Bernard shudders with disgust when he recalls begging to carry Biff's football equipment into the locker room. Bernard's intellect and dedication have paid off; two character traits that Biff and Happy seem to be allergic to.
Bernard has always known that something "wasn't quite right" with Willy Loman, but he never questioned an adult's authority during his adolescence. Now that he has reached maturity, he questions Willy regarding Biff's Boston trip, something the old Bernard would never have done.
Bernard has not changed too much from when he was a boy. We first meet Bernard as a "bookworm" who greatly admires Biff. He is worried that Biff will "flunk math" (Act One), which Biff does. Later, in Act Two in the flashback scene, Bernard just wants to carry Biff's football helmet because he admires him so much. Then when we meet Bernard again in Act Two, he is described as a "quiet, earnest, but self-assured young man." Bernard is on his way to DC to argue a case in front of the Supreme Court; he is a successful lawyer. As Biff has failed and lost his way, Bernard has succeeded through perseverance and diligence.