The Homestretch, part 2

Chapter 25 mainly contains Holden further walking around New York City and a minor descent into madness. I say a descent into madness partially because I've always wanted an excuse to use the phrase and mostly due to his strange, well...

He believes he's going to disappear. He believes he's going to disappear every time he crosses a street, and he calls to Allie (who's dead) to help him make it across the street. While I could not pinpoint an exact disorder, this sounds like a bizarre delusion. Bizarre delusion is a delusion that is completely implausible and strange; believing you will straight-up disappear from existence simply from crossing the street is implausible. Had it merely been a fear of dying from crossing the street (that is to say, a fear of being hit by a car) it would have been a non-bizarre delusion, which is a delusion that is possible, though still false.
Insanity aside, Holden also stumbles upon vulgar graffiti, which strongly effects him (this may have been me reading in too much, but he seemed personally affronted by it), eventually speculating that they'll write the same vulgarity on his tombstone; this sounds as if he thinks people strongly dislike him. This, combined with the apparent personal affront and several other aspects of how he's been throughout the book, sounds somewhat like Paranoid Personality Disorder. Holden asks Phoebe to meet him in the museum so he can return the money she gave him. She comes asking him to take her with him, which makes him feel faint. He tells her no, which makes her mad.