Georgia Ivy Green has several books on male chastity and female led relationships you can find them on Amazon. http://www.amazon.com/KeyHolders-Handbook-Georgia-Ivey-Green-ebook/dp/B00G9PZJRE/ref=sr_1_1?ie=UTF8&qid=1459543230&sr=8-1&keywords=georgia+ivy+green
The two books I recommend are:
Nice dynamic you two have :-)
Have you already considered remote (over-the-internet) sex toys, like the Hush? To keep his mind on you, you could assign him some extra tasks, like sending you erotica that he finds arousing (stories, videos, etc.). I think you can find extra ideas on some books, like this one or this one.
Also, a question: which model of chastity cage is he using at the moment?