In Code by Sarah and David Flannery (https://www.amazon.com/exec/obidos/ASIN/1565123778/donhosek). I would usually target this at bright high school kids, but I think that your kid might be a good fit for it as well from what you describe. Sarah was a high school student learning about cryptography from her math professor dad (David), and presents a great overview of cryptography from Caesar to modern public key cryptography (she came up with an idea using the fact that matrix multiplication is hard to invert for a cipher scheme although it turned out that there was an easy crack for her matrix-based cipher).
I was trying to recall the name of this book just the other day! All the recent CA news had reminded me of it and I couldn't remember the name, but I remembered the cover.
In high school, we had to pick a math book to do a book report on. There were lots of people picking books like Flatland, but I ended up with "In Code". I actually thought it was a programming book because of the word "code" in the title. Turns out I was wrong, and I got exposed to learning cryptography as a high schooler! And since it was a book report, my reading culminated with me fumbling through explaining public key cryptography to my math class.
