Sorry to hear that @longdarkteatime .
In addition to @Araminta1003 's good advice, do find out how he should apply for the A level classes he's interested in at his new school. Classes can fill up, so make sure he doesn't miss a deadline or not get there early enough on registration day. I hope you've found a school which offers the A level combination he wants to do. Existing school parents probably already know all the details, so you need to make sure you find out in time, since you don't have the insider info.
I'm sure that his existing school have taught him lots of strategies to help with any organisation difficulties he has from SEN. It may even (as a pp suggested) help him that the support is withdrawn a couple of years earlier than would have been, whilst you're still there to help him really make those strategies his own. I think that SpLD are often less of a hurdle at A level - since students do a smaller number of subjects, which are aligned to their strengths.
Don't worry that he won't do so well in his A levels and that will affect his future. State A levels will probably help with admissions! And regardless of which Uni he goes to, his strengths - gained over his whole life time - are what will shape his future. You've already given him a huge boost with a great education in his formative school years: that's part of him now.