Ds1 started reading David Eddings at 10, not much adult content in the Belgariad ones, Althalus or the Sir Sparshop ones (Elenium). Terry Goodkind's Sword of Truth books are quite adult and have some very complex themes, DS! is rereading them now as an adult because he missed so much in the early books when he read them as this age.
Has he read Phillip Pullman's Dark Materials books, very multilayered and good for this age. Also William Nicholson's WIndsinger books, Garth Nix Abhorsen books, Anthony Horowitz Power of 5 books, Eoin Colfer's Artemis Fowl books, which even I like, Brian Jacques Redwall books, GP Taylors Shadowmancer series, the Bartimaeus Trilogy, Kevin Crossley Hollands Arthurian books, The Warriors of Alavnna books, SHaron Creech books. My current 12 year old likes the Rangers Apprentice books and Septimuns heap books, also Skullduggery Pleasant, Peter Dickinson novels and Lucy and Stephen Hawking's books about George and his adventures.The boys have also all enjoyed the Manga Shakespeare series.
Factual books: The Number Devil, Can You Feel the FOrce, Tony Robinson's Worst Jobs books, Ripley's Believe it or Not, The Gamer's book of World Records, the A Graphic Introduction to... series, Eureka..., Crisp Packet Fireworks, Penguins Great Journeys series, OUP A Brief Introduction to..... series.