I'd recommend Jonathan Stroud's Bartimaeus trilogy (The Amulet of Samarkand, The Golem's Eye, Ptolemy's Gate), which have a similar snarkiness (and love of footnotes!) to Pratchett.
Then there's Diana Wynne Jones' books: if he's enjoyed Harry Potter, then the Chrestomanci books might be something he'd enjoy as well (the series starts with The Lives of Christopher Chant), or some of her standalone books like Hexwood or The Merlin Conspiracy