I have fond memories of reading Edgar Rice Burroughs' Mars series when I was about 11. Women in servitude, men fighting alien beasts with swords, political intrigue, betrayal. Everything a growing boy needed.

More modern classics are also probably in order. Ender's Game, the Rama series, Armor, The Diamond Age, HHGTTG, etc. And don't automatically dismiss RAH's work because it's too political - that's part of what makes them good. While I wouldn't necessarily start a pre-teen off with the Lazarus Long I've-always-been-too-fond-of-Mom saga, I'd have no problem handing some of his earlier works to a similarly aged relative.