90% is very high to get on an A-level paper- I wonder if the teacher(s) were marking too generously, and perhaps from looking at other papers or the grades across the cohort, they may have now realised this?
The other thing I'd add is that if it was one of the last papers he sad, mental stamina/tiredness may have had an impact?
If the school won't do it, I would imagine there is a reason for it, and unfortunately you can't force them. Perhaps worth contacting the head of sixth form or similar, if you haven't already and explaining the psychological impact on DS to see if he can arrange anything. That said, a B is still a really good achievement, and at uni the style of assessment may be similar- maybe time to start building his confidence up in this way.
In your previous post you did talk about remarks as the reason the school was refusing, so that's why I mentioned them.
I understand it's a really difficult time, but also they may have other students who need reviews of marking etc for uni, and may need to prioritise them? Bearing in mind for subject teachers it is their holidays and any work they do between now and the start of term is essentially overtime for no extra pay...