Holden has the idea that he is cowardly, but when he starts a fight with Stradlater he shows the reader, if not himself, that he can be courageous under the right circumstances. Stradlater is not only bigger and stronger than Holden but one of the school's superior athletes.
Ed Banky was the basketball coach at Pencey. Old Stradlater was one of his pets, because he was the center on the team, and Ed Banky always let him borrow his car when he wanted it.
This is not the only time Holden shows courage. He stands up to the bellhop on principle, refusing to pay him an extra five dollars extortion money, although Maurice is an older man, obviously much stronger and tougher, and Holden is alone in a strange hotel.
In both cases Holden has altercations with men who are bigger, stronger, and tougher than he is, because he is acting emotionally and on impulse.