OK
I think your son has done pretty well not to rebel more than he has, given that the staff seem so far from understanding, let alone meeting his needs.
Kids with unmet SEN tend to play up or get depressed or both. So far you have a bit of playing up, so well done ds.
I think you are right to focus on GCSEs
Most kids with dyslexia have trouble with organisation, so when I was where you are with ds (dyslexia/dyspraxia) and dd (hypermobile and weak working memory so VERY scatty) I needed to get the SENCo on board with two jobs:=
Job 1 - stopping the staff from telling them off for things that weren't their fault
Job 2 - getting help with organisation eg making sure they had a complete set of notes to revise from, making sure they were given homework by email etc
I admit with dd it took a direct email to all staff with a polite reminder that to do less than I was asking for was breaking the law, but the SENCo had had a good try and got as far as she could.
Then with the writing: does DS touch type? If not, I would recommend it from my experience. My ds touch typed all lessons as well as his exams and got extra time for exams as well. Not sure he would be at uni now without it.
BUT it is crucial to get the exam concessions that the school start using these concessions at least in the year prior to the exams.
The school do need you to get an assessment as it sounds like they are not going to be able to organise a piss up in a brewery one. I think the dyslexia institute do pretty reasonably priced ones. If you can afford a full independent ed psych report that would be brilliant as it might pick up dyspraxia and any odds and ends of difficulties which could impact on exams,and ideas to help eg increasing the print size of the exam paper, or having someone to read out text etc.
The aim is to ensure that his tests from here on reflect his ability not his disability and it is at least doubly so for GCSEs.
Good Luck