As I understand it, that's what he did. He open sourced it and kept copyright on it. That would mean that everybody would be free to read it as posted but wouldn't be able to use it or distribute it without permission.
Releasing something under an open source license usually involves retaining copyright but is, itself, giving permission to use, distribute, modify, and distribute modifications. Pretty much every open source project either retains copyright of the work or is composed of the work of developers who each retain copyright of their portion of the work.