Definitely Mortal Engines and His Dark Materials. Also, The Hitch Hiker's Guide to the Galaxy by Douglas Adams and the Keys to the Kingdom series by Garth Nix. If he's an Eoin Colfer fan, The Wish List and The Supernaturalist are both good.
If you want to remind him of how generally awesome books are, I would suggest Inkheart by Cornelia Funke.
Anything by Terry Pratchett!!!
There are loads of books that would be good for him though...
Patrick Ness' Chaos Walking trilogy is very good (each book has been shortlisted for the Carnegie medal and the last one won ), Garth Nix's Old Kingdom trilogy, Historical thrillers such as C.J.Sansom and S.J.Parris are exciting, addictive and also quite informative about the Tudors, the Young Samurai Series by Chris Bradford, Temeraire series by Naomi Novik, Catch-22 (though he may be a little young for that, I didn't read it until yr 11), Paul Dowswell has written some great books such as Auslender, maybe something steampunky such as Scott Westerfelds Leviathan series or Oisin McGanns Ancient Appetites books.
Thats all I've got for now, but I'm sure I will think of more later