When he was in school, he overheard a teacher say he would never learn to read. Hearing his teacher say that caused Grayson to stop trying. So, there he was, an old man who couldn't read, and Maniac taught him to read. Grayson proved that you are never too old to learn something new.
Also I’ve found this The first book that Grayson read cover to cover was The Little Engine That Could. Accordingly, Maniac Magee didn't help Grayson at all. We are told that the latter made an almost superhuman effort to complete the reading by himself. After Grayson was finished, Maniac let out a loud "Amen."