In Philbrick's Freak the Mighty, why is Max convinced he does not have a brain? Is his assessment of himself as a "butthead" correct?

At the beginning of the story, Max is in what he calls L.D. classes, which stands for "learning disabled." Because Max is shy, he does not want to talk as much as the average child. Also, by age twelve, he has gone through a few growth spurts and is physically huge compared to the other students. For Max, shy, plus large, plus bullies who call him names like "Maxipad," all add up to him feeling like he doesn't belong. Incidentally, Max uses the term "butthead" to describe the student in L.D. class who actually called him "Maxipad" (3). He also uses the term to describe himself when Freak first asks Max if he has ever heard of King Arthur. Max thinks to himself, "The only King Arthur I know is the brand of flour Gram uses, and if I say that I'll really sound like a butthead" (16). Hence, Max uses the word "butthead" as a way to call a bully a name, but also to call himself dumb.

Later, with Freak's help, he learns that he's not a butthead or dim-witted, he just needed someone to believe in him. Freak helps him to appreciate words, the dictionary, and reading. Max eventually discovers that he's not as learning disabled as he was pegged to be. In the end, it is actually Max who writes the story, showing that he was able to overcome any learning obstacles he once faced.

"I never had a brain until Freak came along and let me borrow his for awhile, and that's the truth, the whole truth. The unvanquished truth, is how Freak would say it" (1).