This morning started badly but ended much better, I'm now out for most of the day.
DH was furious at DS for not having done something he thought he'd asked him to do, I managed to referee until we got to a point where there seemed to be agreement it was a misunderstanding and DH apologised... a turning point was when I very gingerly asked him to rephrase the accusatory tone of a question to DS, and he did (honestly it's always anyone's guess as to how he'll take things).
And we had a honest discussion about how DS has to find a way forward (with our help) with effective ways to revise because we do appreciate that remembering what he is taught is hard for him, so he will need lots of reinforcement. DH also said he understands that DS will not do well across the board at GCSE and that's ok, ie subjects like English literature will be exceptionally hard for him, so that was good to hear he understands.
I think the next thing really, as we have been promised more sessions with counsellor, is to talk to her about how DH can reframe his approach from 'him vs DS' to 'us all supporting DS'. Taking it all so personally doesn't help DS, makes DH and me less able to help DS and less effective at doing so. I'm not saying DH can't be frustrated and worried, but the way he directs it is the problem.