Symbolism, Imagery, Allegory
We'll let Freak take the lead on describing King Arthur:
"Arthur was this wimpy little kid, an orphan, and there was this magic sword stuck in a big stone […] All these big tough dudes came from all over to yank at the sword and they couldn't budge it. One day this wimpy little kid tried it when nobody was looking and the sword slipped out like it was stuck in butter." (4.9)
Hmmm. A wimpy little kid who everyone underestimates because of his size. Sound familiar?
Freak loves thinking of his adventures like those of Arthur, and in a way, Arthur's story symbolizes Freak's hope for the future. While he never explicitly talks about his hopes, Freak's fascination with King makes us think he wishes he could grow from a wimpy kid with a strong heart, into a brave, strong knight.
And although Freak passes away, he does serve as a knight in shining armor to his friend, Max. Freak brings a whole new world to Max—all of which stems from books, especially King Arthur's stories. And at the end of the novel, Max says, "And now that I've written a book who knows, I might even read a few" (25.19). Freak's love for books and stories has opened Max up to possibilities he didn't even know existed before.