Explain to him that the exams are non-negotiable. He has to do them. And if he fails them, he'll only have to do them again. And again. Because all jobs require a basic level of education. So if he doesn't like them, the best solution is to work now, so he never has to do them again.
I'd tell him to focus on Maths and the two Englishes and maybe a couple of other ones that he likes best, and just not worry about the rest. Way better to get good grades in the most important subjects than to get stressed about trying to pass them all and failing.
The school should have spotted dyslexia ages ago. Shame on them if they didn't. If he really does have it, I might suggest that he doesn't take the exams, but gets a proper diagnosis and adjustments, and then just does the key subjects as resits next year, if that is an option.
In terms of helping him - the best way is to ask him to explain stuff to you - to check he understands it. There is a massive gap between familiarity and recall. Teens don't always realise this. They read or hear something and recognise it and think 'Oh yeah, I know that.' But that's not the same as being able to recall it with no prompts in an exam.
With Eng Lit watch all the versions you possibly can. If it's Inspector Calls, the BBC version is pretty good. Macbeth - the movie and maybe try to see the play too. Get hold of the full text comic strip version too. That's helpful for dyslexics to follow the story. Christmas Carol - loads of TV and movie adaptations and often on in local theatres. Not all versions will be 100% faithful to the original, but he can get to know the main characters and story lines and then use BBC bitesize etc for the rest.
With Maths, Khan Academy free online tutorials used to be good (years since DS2 did GCSEs so I might be out of touch) but Khan is good at explaining things clearly to people who struggle with maths. Pick a topic he needs to get better at and help him work through it.