In fairness to Stradlater, how does he first react to Holden's comments?

Stradlater knocked Holden down and knelt on his chest in order to put an end to the fight. He doesn't want to fight Holden, and his actions give Holden a chance to pull himself together.

As far as Holden's comments....... Stradlater makes them a joke. He isn't going to say anything, so he throws in some humor. He had no idea that Holden was going to throw a punch or take offense.