webwoman can't really help with the strategies, as we still have it all to come (ds1 is 13, Y9 and has ASD with slow processing), but I can relate to the anxiety your ds is feeling re the GCSEs.
You mentioned the TA recording his homework for him and that's the one strategy we've insisted on since he started secondary, as ds's writing is illegible. Can your ds type? Mine uses a laptop in class and will be allowed to for his exams too, as it's his normal method of doing his work. It might be worth considering.
As for the anxiety. Ds started panicking about GCSE's last year. We took the pressure off completely and told him that all we want is for him to go to school every day, do his best and whatever his results are, we will find a way to make things work for him (as in to get into the field of work he's interested in). We told him that nothing is worth getting that anxious about and actually, GCSE's aren't the be all and end all, they are just one route forward. We then explained that it might take a little longer and require a longer route to get there, but that doesn't matter and we will be here to support him every step of the way. (It helped that I'd been doing a bit of reading around about college courses, apprenticeships etc online, so could tell him about some different options. There's actually a lot more available out there than I knew about, so it was worth doing a bit of research to set my mind at rest as much as ds'.)
He's a lot happier about it, at the moment at least and his grades have actually gone up since we took the pressure off. Not sure what will happen when he moves into Y10, but we will just keep up the same dialogue about just going to school, doing his best and seeing what happens, then we'll work it all out from there.
I get so irate with how hard children are pushed regarding exams these days, the pressure on them is unbelievable and so unnecessary, when, in reality, there are lots of different routes towards working and having a rewarding career.
I hope your ds is feeling a bit better about it all by now. 