I'm looking for any and all organ sheet music for the traditional Latin "Vidi Aquam" chant (which is used instead of the Asperges Me during Eastertide.)

A few important points:
1. I already have the music from CCwatershed.org. We're looking for something with a bit simpler accompaniment. It's for a 10 year old boy to play.
2. I know that Google is my friend, but I already tried Google with no success.
3. Unfortunately, my budget for this is about $0. Our chapel is already in debt at the moment. So I'd like a scan of someone's organ sheet music, or a free PDF, unless someone wants to donate :)
4. It has to be in regular "modern notation". We already have the Liber Usualis, Gregorian chant "square note" version. That doesn't give any notes or chords for accompaniment, but just the melody line. Our son can't yet play accompaniment just by looking at square note chant notation. (That would be lovely, of course, but he's only 10).

What we'd REALLY like is sheet music where the (guitar, named) chords are written over each chord. Every time the chord changes, you have a small "F", "Bb", "C", etc. written above that note. Some sheet music has this, but about 90% of it doesn't.

P.S. We're also looking for Mass IX, "cuм jubilo", for feasts of the Blessed Virgin. It's one of the most beautiful Gregorian chant Masses, but right now we don't have chords for it so we can't use it during Mass.

Would it help if I write chord symbols (like Jazz lead sheet style you describe) over a neumes (square note) melody? Or can he not read square note at all? I can also write chord symbols over modern notation. You could also have him play the ones on CCWatershed but simplified.

One thing you might consider is having him try out other methods of accompaniment. For example, holding a drone bass on the tonic of that mode. Let's say he's accompanying Missa Orbis Factor (mode 1) using "D" as the tonic, in other words, it looks like he's playing in D natural minor so the incipit of the Kyrie is "A Bb A G A D". Since "D" is the tonic of D dorian, he can just hold a "D" in the left hand for the entirety of the chant. Yet another option is for him to try an organum accompaniment wherein he would just accompany the melody by playing in parallel fifths or sixths or thirds, etc. So for example, Kyrie Orbis Factor to accompany "A Bb A G A D" in the right hand he would play "D Eb D C D G" in the left hand (perfect 5ths).

He uses modern notation right now. As long as the chords matched the key the particular sheet music was taken in, that would be great.

He actually does a pretty normal job of organ accompaniment. He just doesn't like having a different chord for *every single* note.