The first choice is "The Mighty" by Sting which was written for the film version of the book. It literally describes Kevin and Max with reference to King Arthur and his knights.
Walking this high road are warriors from an olden time
. . .
And no man ever dared break our stride
Or the brotherhood that binds us.
Think about songs which deal either with the characters themselves or the situations and themes in the book. Brotherhood is definitely a theme in this song. Come up with some key terms such as brotherhood or courage and google them: "songs about courage".
I did this ("songs about brotherhood") and found a song by Alice in Chains called "Brother," and reading the lyrics, it sounds like Kevin giving Max advice.