Does anyone know the title or author of this book? Please help!?

Okay, so I read this in school once and I don't remember a lot, but here is what I do remember. I'm pretty sure it was in two points of view, one was this disturbed sort of kid and the other I think his grandparent or somebody who lived through a war and can't speak, and the last word he was able to say was "I" and then he wrote on the walls. Like his vocabulary was slowly dwindling until he could only say one word and then nothing at all. There was a flashback scene where everyone was dying in the street and he had to get to his wife and everyone was crying out for water. In the story, the kid finds this piece of paper (I think) which has a clue and I think a name on it, and he has to track down the clue. So he sends his child psychologist a letter from his mom saying, "I will keep sending you checks, because you are a nice person." Then he goes through the city looking for clues. I don't remember how it ends. Here are some quotes I remember (may be slightly paraphrased):

"I know a lot about birds and bees, but I don't know a lot about the birds and the bees."

"They say when you look down on people from a tall building, they look like ants, but they don't. They look like miniature people."

Sorry if this isn't a lot but it's all I remember. I also seem to remember it being fairly long and depressing. I think the kid was a tween and his mom was a single parent.