A bit on the lighter side, I do recommend The Man Who Counted which I read as a kid and absolutely loved

https://www.amazon.com/Man-Who-Counted-Collection-Mathematic...

I read the original in Portuguese but would assume it's just as good in English, given overwhelmingly positive reviews on Amazon

See also https://en.wikipedia.org/wiki/The_Man_Who_Counted

It won't really teach him math per se, but if my experience is any indication, it will get him hooked on developing intuition and he'll find beauty in otherwise mundane topics such as arithmetic. It's an incredibly engaging story aimed at younger readers but fun for people of all ages – think Arabian Nights with a character that loves math.

Come to think of it, I've got to buy it again and re-read it one of these days

Whatever it might have been in English, the title of his book in English is The Man Who Counted: A Collection of Mathematical Adventures,[1] and the news report in English kindly submitted here refers to the book by its familiar English title. (I have known of this book since before Hacker News existed.)

[1] http://www.amazon.com/The-Man-Who-Counted-Mathematical/dp/03...