Books by M. Gardner make superb reading. I like S. Barr's Experiments in Topology as a nice introduction into a kind of mathematics that is different from the school variety. Many math history books show mathematics as a human, evolving branch of knowledge. I would look into E. T. Bell's Men of Mathematics or J. Nuir Of Men & Numbers. But there are many more that your boy might like. Logical puzzles by R. Smullyan do not require any math knowledge.

You may want perhaps to get a copy of, say, E. Kasner and J. Newman's Mathematics and Imagination that covers many different topics and see which ones your son is more curious about. Perhaps you can read and work out the books together with your son.