Reddit reviews The Paper Airplane Book

The Grapes of Math and the other books in that series are pretty good. Kind of a mix of numeracy and recreational math, but for kids.

There's a good book called Innumeracy, but that may be over the head of a child that young.

Once he's had a bit of algebra, A Long Way From Euclid is an excellent book. Even without the algebra, he might benefit by reading it together with someone.

You may also want to look at the library for introductory material on Euclidean geometric constructions, orgami, tessellations, fractals, topology (Flatland?), stereograms, Logo ("turtle graphics"), combinatorics, celestial mechanics.

Or for more hand-on stuff: play with Newtonian physics - build stuff with catapults, marbles, etc. The Art of the Catapult is a good book with plans for various machines.

Also, building kites, or paper airplanes:

The Paper Airplane Book

Paper Airplanes and Other Super Flyers

The Great Kite Book

25 Kites That Fly

Kites For Everyone