Hi OP, I have a DD in Y7 with a diagnosis of dyslexia and hypermobility. Her VR and NVR scores are high (above 98th centile) but processing speed is average and working memory is only 7th centile - so very spiky profile.
She was diagnosed at the age of 7 (it was rather horribly obvious so school organised everything as soon as she was old enough) and was retested at 10. She's probably 'moderately severe'.
The absolute game-changer for us was moving to using a laptop (the hypermobile finger joints made it incredibly painful and hard for her to write so she avoided it as much as possible which just compounded the problems). Ignore the PP who said not to go with a laptop. Over the last 5 years we've had a lot of advice from experts in dyslexia and every single one of them recommended a laptop as soon as we could get her touch typing.
We did some intensive online touch-typing courses (English Type and English Type Junior are both dyslexic friendly and DD was very motivated by the prize systems to really engage with the course) and now she is a different child when it comes to writing.
We're planning to have a serious look at speech to text software this year (Dragon Speak or the Office 365 one) and then focus on editing skills. Her spelling is always going to be dire so it's looking for work arounds as much as solutions - but she now loves writing and instead of 3 painfully produced sentences, the teachers at least now get 3 enthusiastically produced pages.
The 'usual way of working' is going to be the issue you have for GCSE as he doesn't have things in place yet. I know my DD will be able to have a laptop and various other bits of tech for exams as that is how she is in class. You should aim to start 6th form with as much of this in place as possible so you can have those for the exams.
Even a diagnosis and being able to wave bits of paper doesn't mean you won't have to fight for lots of things. DD has never yet been given an extra 25% for tests and assignments... which was more of an irritation at primary than it is so far at secondary, and we have had a couple of teachers ask DD to do a test assignment on blank paper by hand (ludicrous as she is then doubly handicapped as she doesn't ever write by hand any more, and there was no reason why she couldn't have emailed the assignment at the end of class). You can also get some kids saying it's unfair that A gets to use tech - although less likely at 6th form I would imagine.
What you get in terms of access arrangements will also depend on the severity of the scores alongside their normal way of working.