Drained I totally understand the frustration! Firstly, if he has dyslexia, you do have to help him get organised at this stage and you’re doing the right thing. There are too many GCSEs for him to do all this on his own and dyslexia is exhausting anyway, so he’s already feeling like it’s a mountain and doesn’t want to engage. Quite a few students would rather not attempt something at all than struggle because it makes them feel bad.
However, Mmzz is a genius for remembering the agile thing - breaking it down into small tasks and then moving on definitely helps. You could even apply this to the practice papers- decide to do certain questions or section on a practice paper. Work out how long he should spend on the question (including the extra 25 per cent if he gets it) and say I just want you to do this, nothing else, but under exam conditions so no phone. As soon as it’s done, review the answer - any learning points write them down for him but it’s done, phone back, revise those points next time, move on to something else. It’s a psychological thing but if you set a timer it works. Now, if he’s really resistant about writing, set a timer for 5 minutes, write a paragraph, then another 5 minutes etc till you have enough for the answer to the question. Then stop.
I don’t know if this will work but I’ve had some success with students and reluctant writers!