Well, of course the bellboy bullies Holden. He cheats him, having first quoted one price for Sunny's services and then asking for more. (And Sunny's name is sadly ironic!) He represents what Holden secretly feels about sex; that is is low and dirty.
Holden has this idea that sex is unrelated to love. He loves and idealizes one girl, who is having a bit of trouble with her overly sexual stepfather. He gets mad at his roommate when he believes the roommate may have made a pass at this girl. At the same time, Holden wants sex, he wants to lose his virginity, but he despises himself for his own desire. He dislikes the fact that he is attracted to girls for whom he has no real liking.
Also, Holden is a tall guy and there's no reason to think he can't fight, but he is excessively timid. He can't assert himself, usually. He thinks things, but never says them and rarely acts upon his beliefs, for example, with Ackerman. He has to be really annoyed to say anything, and when he finally does, he backs down immediately. The bellboy threatens Holden and Holden gives him the extra money, crying as he does so because he is ashamed of himself.
Therefore, the bellboy/pimp, who trades in sex from sad little ignorant Sunny, is both the representative of all that Holden hates about sex and about himself as a weakling.