
I think by Friday, whatever career you're in it feels like you've done it for 325 years.
I think maths is quite unique in being a subject people can develop quite an every day fear of, or be fine with it every day but then flap in a maths test and be unable to work things out that they can do perfectly easily on a normal day.
I have a child who is the latter and described it perfectly arriving home today after a test on the whole year's work last lesson on a Friday in KS3 having flapped in some questions she could have done perfectly well - but at least then having the maturity from past experience of this to move on and do the rest. Rust from illness would not have helped.
One of the things I think helps to prevent this kind of situation is "overpractice" i.e. go further than just understanding and doing it through to being able to do it standing on one's head. It's like preparing for a performance - particularly if one has "stage fright". If it's second nature, it is easier to do it in the situation where one feels a bit stressed.
All the best OP - it sounds like your son is good at maths but just simply lacking in his fast recall of number facts at the moment. It doesn't sound like a widespread and generalised maths anxiety.