I have an ADHD/ASC son. Similar to you he was failing. There are things you can do so ignore people saying there isn't anything you can do! However it's unlikely to be without pain for both of you ime.
I couldn't sit and watch him fail. He is v bright but couldn't organise himself and focus or understand the benefit of revision (cause and effect were tricky concepts for him).
So I:
-Went through his syllabuses and made a revision plan - very structured with what he would cover in each section and timed with adequate breaks.
-Agreed he would try to go out during at least one break at day and get fresh air and exercise.
- Printed out TONS of past papers. Marked and went through them with him. He did them multiple times (this helped with his tendency not to explain context).
- he sat at the kitchen table to revise so I could make sure he didn't get distracted and for most of the time I literally sat with him.
- physically removed anything distracting.
There were a lot of tears (from both of us!) especially at the beginning but once he got used to the idea he was OK. It taught him how to revise, the point of revising and helped him focus (we did short sessions with lots of breaks).
He went from failing all his mocks to getting all As and Bs so I can confirm it worked - he got 100% in some of his science papers which was amazing and slightly unbelievable!!! He also needed much less support in his A levels and he went on to do a degree which I barely helped him with at all so the experience was a learning one rather than support he needed on an ongoing basis.
However it took up ALL my time out of work for weeks and weeks. It would have been much harder if I'd have had other children.
I have inattentive ADHD too. I think I hyper focussed on it which was probably an asset in this circumstance. But it also meant I understood what he would be struggling with and what might help him.
Novelty, internet and urgency are the three key words to kick in the ADHD brain. Accountability also helps.