PacificMouse - you are right - there is no good answer. It is the exact reason I've tried to avoid it so far.
To adopt the phrase used by the commentators about the Greek debt crisis until this summer, I won't solve anything by teaching DS GCSE maths early, I'll just be kicking the can down the road.
The situation last year (Y8) was that DS learned very little in maths lessons because he was waiting for others to catch up.
Now, in year 9, he's still waiting. He says they have been doing work from the level 7 curriculum this term, which takes no account of the DC, like DS who were level 8s last year, having mastered level 7 stuff at least 18 months ago. Obviously they will hit level 8 at some point, and probably there's something that DS could do with a bit more practice on (trigonometry, I think), but given that he got an 8A after just a few hours of teaching, I seriously doubt that the classwork will challenge DS even once this year.
If DS could just hold on a bit longer - 3 years more - then undoubtedly the pace will pick up, but that's a long time in a teenagers life and he's worrying me by becoming disengaged. He's been bored for a while but the disengaged bit worries me for obvious reasons.
So, I'll teach him at home and maybe he will do a GCSE privately, or maybe not. Its up to him really.
It means that year 10 will be a revision year at school, as will year 11. But so what? No one seems to think its a problem to make all the classwork revision in year 8 and 9, so why should they bother if that's what year 10 is too?
In DS's case, it will just mean that he can relax in maths during the GCSE years and save his energy for working in the subjects that his dysgraphia seriously effects.