I read this novel c. 2010. The main character's name was Nathaniel. In the beginning of the book he was in a car accident, and I believe the driver (his grandfather?) was killed. Nathaniel ends up being placed in some sort of psychiatric care, where he discovers that the accident (through a concussion?) has unlocked a talent for math, or perhaps just given him synesthesia? I don't really remember what the main conflict of the story was. I do remember that near the end of the book he and some of his friends from the center are AWOL and one of them, a kid on crutches, climbs on top of a parked car and "conducts a symphony" of car horns with his crutches. I remember the last chapter ends with Nathaniel and his love interest finally sleeping together and he says that when she says his name it appears in the air in front of him (synesthesia). But in the epilogue they break up. I'm pretty sure the whole thing was told in the first person.
I don't remember it being a particularly good book, but it's bugging me that I can't remember what it was called and can't seem to find it.
(There's a scene I remember that might not be part of this particular novel where the main character spends some time in a sensory deprivation tank and does not score with the pretty girl he meets at the hippie hangout said tank is attached to.)