Not a book but you could construct a little walk through some historical crypto as fun games to play. Caesar cipher, Vigenere cipher, one time pad, then learn about primes and make some repeating keys..you could do a super simple matrix example to make tiny DES s-boxes. I bet claude could come up with some games / fun little exploration things to do together, and you and your son could do a lot of fun motivating math together. You could anchor it with rewards like pictures of you and your partner or some family history or stuff that feels like a treasure hunt.

Mine are out of the house, but I had a few who might have really liked doing something like this.. could make a nice Christmas present, the first mysterious letter..