I'm listening! OT, but if your DS's interests extend to physics, it might be relevant to recommend the Uncle Albert books such as
http://www.amazon.com/Uncle-Albert-Quantum-Russell-Stannard/dp/0571226809/
and the books by Lucy and Stephen Hawking such as
http://www.amazon.com/Georges-Secret-Universe-Stephen-Hawking/dp/1416985840
Even more OT, if he's into chemistry you might want to look at
http://www.amazon.com/Disappearing-Spoon-Madness-Periodic-Elements/dp/0316051640/
which my DS6 has just devoured - this one is anecdotes, not a fictional story, and some might consider some of it unsuitable for children.

The closest I know to a story-based book with much higher level maths than the ones mentioned is unfortunately still a draft by a colleague of mine, seeking a publisher, and only a handful of IRL readers are getting access to it just now. If this changes I will certainly publicise it here, since DS loves the draft!

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