I don't truly have direct experience teaching somebody progressively at the piano with this, but I am generally very willing to try things, especially in situations that really call to me as a teacher. I have observed some private work (not piano) with severely autistic children, and of course there has seemed to be quite a bit of "rewarding" that goes on. The reward can be anything that feels like a reward to that student. And then repetition and persistence on the teacher's part. Years ago I was considering a job as a private teacher for severely autistic children, in fact, so my observing involved a lot of mentally putting myself in the position of both the student as well as the teacher. If I were in your situation though, I would basically give myself permission to think WAY out of the box.
For example, if note reading ever does come into play, presenting it in the form of art and pictures instead of regular sheetmusic. Working with blocks to represent concepts at the piano. Working just to get them to respond (in *any way*) to music in general, listening, dancing, drawing ... *anything.* Using little animals at the piano who they control in finding notes, etc., vs. them being asked/controlled to find things. Let him play the actual piano strings. As a child, I used to regularly take the piano apart (and put it back together) ... it was an upright, which I managed (out of sheer love and curiosity for the sound) to take as much casing off as possible (as in, the entire front), and then I'd play. Let him be fascinated by how it works.
When a pattern is discerned, magnify that pattern in whatever way you possibly can. Bring it into various mediums so he can see it from various angles and not just as trying to sit at the piano bench, but so that he's getting the concept and idea of the pattern itself, not just about sitting at the piano. There are life size teaching materials like giant staffs, giant floor pianos, etc.. Stories. Everything in short spurts until *anything* is gained, rewarded, then moving on, perhaps revisiting a subject that same lesson. hmmm ... my mind is going nuts with this now, so I'll stop!