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).