Ds isn't dyslexic but he does have vision problems, so has always struggled with reading and writing.
If your son uses a computer for all his work I suggest he sets up a template with seperate spaces for the subject, the objective, the stated learning outcome, and then bullet points as he goes through the lesson. DS does this and then he can reorder the bullet points, expand on them and build up to an essay. It does take practice but it really works. He is currently writing his gcses and expected to get A/A*, B and C. He wants to take history, economics, philosophy and eng lit!!!! For a levels, so it does work but takes practise.
He also likes mind maps very large, I get them printed on A3 and we stick them everywhere. Also he has flash cards for me to ask questions, basically plain postcards and a fact on each, these are added daily, as in today it's a formula for something in science, a history date or treaty, etc. All in random order. Also we have both recorded podcasts for him to listen to. Pages from science history and English books.
Good luck, it does come together with time practise and patience.