a nine-year old who loves chant

so, I have a piano student who loves chant. And he is nine years old.

As part of his lessons, I have been having him learn a different chant every week (written in modern notation) on the piano keys.

His mom told me that his dad got him a copy of this book on chant, which he has been devouring. It seems pretty legit, but I only got to look through it for a moment (and I hope it isn't too "new-agey-spiritually" if you know what I mean.)

But...then what? He is homeschooled, and with very supportive Catholic parents, but keep in mind I'm only his piano teacher, so I don't really have time to do much more outside of things directly related to piano. I'm thinking I will next have him start learning chants on the piano that are written in square-note notation. He has a very good ear, and will sing along quite well with what he is playing on the piano, so of course the obvious benefit is that by learning how to read chant notation better on the piano, he will be able to sing it better. But of course that will only get him so far, and at some point he is going to have to learn the practical application within a choir, and how to make it actually beautiful, (by following whatever method of singing chant that is being used.) He is also VERY creative, definitely a budding composer (but he is only at level 2 in the piano books, so his composing skills on paper are kind of limited to that.)

So...any other book resource ideas? (keep in mind that it has to keep the attention of a creative nine-year old!) Or other general teaching ideas?