Let's first clarify a point about this. Bernard does mention to Willy that he is on his way to Washington D.C to try a case. What he does NOT tell Willy is that this is a big deal case, in fact, big enough that it will be tried in front of the Supreme Court.
This part of the trip is specified by Charley, and a discussion ensues between Charley and Willy which is a pattern between them already. Often, their conversations go back and forth about what if's, the constant giving of money to Willy (by Charley), and Willy's refusal to see exactly where he went wrong as far as his children, his family, and his life.
Therefore, the phrase
"He don't have to-He's going to do it"
was a phrase said to Willy by Charley when Willy began to lose his temper and, in a combination of jealousy, awe, and admiration, he recriminates Charley about Bernard for not telling him the extent to which this case extended.
When Charley says that phrase is as a reaction to Willy's constant behavior of talking and not doing. Hence, Bernard is different: since he is a "doer" and not a talker like the Lomans, there is no need for him to bragg to Willy about his good fortune.
"He don't have to-he's gonna do it." (Act two-page 95)