Did you get his mock papers back?
I think it sounds like something else went wrong - getting 32 marks on paper 1 and only 5 marks on paper 2 is a much bigger difference than usual especially when paper 2 has a calculator.
More than 5 marks would be available on paper 2 for really basic questions that he's have learned in primary school and would likely be able to do if he can get almost half of paper 1 right. It sounds like some combination of panic, low confidence and giving up, just not answering questions, etc, rather than actual inability to get more than 5 marks.
So I think that's where to start - get his paper back and actually see what went wrong, and then you can work out how to address it. Some schools just give an analysis of which questions were right/partly right/wrong, but that isn't going to help here - his might say things like he got a question on addition wrong, but you have no idea from that whether it is that he didn't try to add, didn't use a calculator, made copying errors, didn't answer it at all, etc., and you need to know that before you spend time practising adding, for example. It sounds like he needs to work on confidence, strategy, not giving up, making sure he's read the question carefully etc
The maths genie site has practice papers that are split in half, into 45 minute papers, and those can be useful for children who find the full paper overwhelming to even start.
But I'd imagine that if he can get 32 marks on the first paper, he should be able to get relatively similar on the others, and that's a very different prospect - in 2025 (edexcel), he'd have needed approximately 48/80 on each paper to get a 4 overall, and while that's still a way to go, it's not as insurmountable as his current marks might first appear.