why would you snitch on him, its really none of your business what hes doing, how does it affect you if hes cheating, its his life, get on with your business and focus on your own exams and how you got through alevels honestly without needing to cheat, eventually he'll probably get caught, plus you dont know why hes doing it, how does it affect you or concern you in any way, if its not from the fact that your jealous lol
ISAs are as every bit important as each of your science modules as they each count for 25% of your final GCSE. If he would cheat in an exam room, he would get disqualified. So why should he get away with it for ISAs? I think you should report him as it crosses all moral and social boundaries.
Just wondering of you'd considered the possibility that he has just claimed to fake dyslexia because he's embarrassed? Some people mightn't want people to think they're dyslexic.
Think about what will happen to him if you report him. You will at least be partially responsible for that. If you don't feel like being that guy, then leave it. Most universities have interviews anyway.