Looking for a Book

I have some trouble finding a book. My father read it to my brother when he was younger, but he can't remember the title. I was wondering if anybody could help me. The book is about this boy who runs away from his parents and lives on the streets. Years later he comes across 8 Mile (I presume), the line which seperated the whites from the blacks back in the day. He, himself, is white and is looked down upon when he crosses into the black side, but he doesn't know what's wrong. There's something more about the plot, I can't quite remember. I do know that he is given a gigantic ball of string, a knot actually, and nobody has ever mastered to untangle it. It takes him two days to untangle it and so he wins a year supply of pizza. But he's allergic to peparoni, or cheese.. or something that was on the pizza. I also remember that he was taken in by a woman, she asked him why he never slept on the bed she gave him. He said he preffered the floor because of all the years he's slept on the street. He said the bed felt like "mashed potatos."
This book could be from six, to ten years old. If any of this information rings a bell to anyone, please respond. Thank you for your time.