Ah, the dark dank hole from which our hero must emerge. Well, it's really not that dank and probably not that dark, but Max still has to pull himself out of it and come back to the light.
The down under is symbolic of the sadness that has surrounded Max for as long as he can remember. Hiding away makes it easier for him to avoid reality:
I'm still living in the basement […] but do I complain about the crummy paneling, or the rug that smells like low tide? I do not. Because I like it in the down under. (2.1)
Being confined to the basement is something Max chooses. And that means he's the only one who can choose to come out, too.
Freak, of course, helps him out, luring him from the basement to go on adventures and quests together. When Freak dies, Max runs back to hide in the dark again, but even though he's not alive, Freak's legacy (the book) draws him out again. And as we read Freak the Mighty's story, we are left with the hope that Max has once and for all come out of the darkness.