Bod has already shown great courage in escaping the evil clutches of the man who killed his family. So when he comes to meet the Sleer in the graveyard, we shouldn't be too surprised at how brave he is.
Even so, Bod can be forgiven for feeling more than a little nervous as he approaches the Sleer's den. The young boy is determined to buy a headstone for the witch Liza Hempstock. To pay for the headstone, he's going to steal some of the treasure that's been guarded by the fearsome Sleer for centuries. The Sleer is not a creature to be messed around with, so Bod is either remarkably brave or remarkably foolish for even thinking such a thing.
As he walks down the stone steps into the Sleer's dark and dangerous lair, Bod can feel the hate and greed in the air. But he's not scared, and remains focused on getting Liza her gravestone. Even after the Sleer tells Bod that nothing ever leaves, the young lad is undeterred. He grabs a brooch from the Sleer's horde of treasure and runs up the stairs with it, leaving an angry Sleer in his wake. If that isn't bravery, then nothing is.