If he's already engaged in cryptography, feed and nourish that.

Once he's getting some diminishing returns, I recommend Computer Science Unplugged: https://classic.csunplugged.org/documents/books/english/CSUn...

It teaches the basics of information theory and computability with a series of lessons and paper games (including some ciphers and why they work). All of my kids went through it, and had fun doing it.