This sounds really hard.
I wonder if maybe the disability office isn't communicating properly with the department?
I teach students with disabilities (and I was one myself), but occasionally I find the disability office has merrily told students that x or y will happen, without actually checking that it's a reasonable adjustment, or even something we can do.
To give you an example, recently I had a student who'd been told they were entitled to 1) a verbatim transcript of my lecture, 2) a summary of the content of each lecture at the beginning of term and 3) the handouts emailed to them. The third requirement was easy - the other two were really not, because most lecturers don't speak from transcripts (ie., no transcript exists), and because I didn't know what each lecture would contain until quite soon before I gave it, because I tailored it to the needs of the students in the room.
It was quite easy to explain this to the student and to provide alternative support - but it did make clear that there can be miscommunications even when everyone is keen to help.
I do see that the situation here may be very different, but I mention it because it brought home to me how easily people might promise something that seems obviously possible to them, which actually isn't appropriate.
Aside from precedent, what is their argument? Are they providing any alternative support? Can they justify what they're doing, and can they show they are being fair to him?
I'd start in by asking that. It may be you get nowhere - and there's only a small chance it's something like my situation, where there's been a genuine miscommunication somewhere - but if nothing else, their response should show clearly whether or not they recognise he deserves a level playing field.