Holden has the highest opinion of his brother D.B. as a writer. Holden is also a pretty good reader, has read many books, and does well in his English classes; so, he feels like he can be a critic on the issue. Apparently, D.B. wrote a collection of short stories and called it "The Secret Goldfish." Holden thought it was really high-quality; therefore, he thinks that D.B. is doing something beneath his abilities by writing in Hollywood. Not only that, but he hates the fact that his older brother is writing for movies because he also hates movies. Holden doesn't believe that actors are authentic, but complete phonies. Everytime he goes to a movie, he can't accept the actors as the characters they are playing because he can only see the actors as themselves. Overall, Holden is very disappointed when D.B. backs away from writing literary works for writing superficial stories for movies.
Another thing that Holden doesn't like is that D.B. has a lot of money for writing movies. He says the following:
"He just got a Jaguar. One of those little English jobs that can do around two hundred miles an hour. It cost him damn near four thousand bucks. He's got a lot of dough, now. He didn't use to. He used to be just a regular writing, when he was home" (1).
When Holden refers to how much money D.B. makes, this is probably where he makes the connection to his brother being a prostitute. He gets paid a lot of money to do something degrading and obscene in Holden's book. Therefore, D.B. has sold himself like a prostitute.
It means that his brother is out there being a sellout. According to Holden, the entire world, especially people who love appareances, looks, and the superficial (namely, the Hollywood society) are phony, fake, and Holden hates them. At first he was a huge fan of his brother for his writing ability. Holden held that quite dear. When the brother got a chance to make movies Holden felt that he was just being a sell out and a fake like the rest.
DB is Holden's brother who started out as a small writer and became a Hollywood screenplay writer. Holden says his brother "sold out" to Hollywood. He felt that his brother's stories were better before he sold them to Hollywood. Holden is the ultimate hipster in this scenario and that's why he calls him a prostitute.