Tutor who does exam practice.
Here are lots of exam papers:
Edexcel Maths A-level Papers - PMT
Both practice and real ones, and the mark schemes with them.
This time last year ds got an E in his (admittedly totally messed about by his maths teacher) mock paper.
He'd got a (not worked hard at all) 7 at GCSE.
When he got a D/E in his February Mock, dd1 started teaching him.
She'd see him once a week. Initially she went over the syllabus. Then by March he would do a paper a week, she'd mark it and give him corrections and some extra stuff to do according to a question he'd found hard.
He got about 20-30 in most papers. By just after Easter he was getting 30-40, and both were feeling disheartened that they'd done so much work, for so little improvement.
Then start of May he suddenly got 60. Dd actually asked me if he'd done it in his room and just googled the answers. he hadn't; he'd done it downstairs so I could see him.
He had another couple of weeks of hovering around 50, then suddenly he was getting 60-65 consistently.
In the real thing he got 80% on his best paper, an average over 70% across the papers and overall an A.
What changed?
There were a few things.
Firstly when he started doing the papers, he'd loose interest/concentration after 60 - 75 minutes. Just doing the papers helped build his concentration up. (he does have ADHD, and ASD) They had hour lessons at school, so the only time they did papers of more than an hour the the actual mocks.
Secondly getting him to see that just because he couldn't do part a, didn't mean he couldn't do part b, c and d. So he went to from taking one glance at a question and saying "I don't like surds" and not doing anything to thinking "well I'll do part b, and then he'd often find he could do part a after all.
And lastly, writing. You can pick up a few marks by writing a few sentences. If they ask why or how or conclude, often there will be a mark for simply writing at the end something like "answer is correct".
Ds thought this stupid, so wouldn't do it.
So dd worked on him looking at a question and seeing what he could do. Putting things down in words and seeing that gave him a couple extra marks.
So he went from looking and say doing:
Q1, 2, 3a, 6, 7a, 9a,b and 15.
To doing at least part of every question. So his potential score went up from say 40% to 95%.
A lot of what he learnt was exam technique. But also he then went into the exam with the confidence that he could do most of the questions so approached them in a "I can do this" rather than a "Can't do them."
For your ds, have you also asked for him to be assessed as slow processing, so he might get extra time in the exams. He will need to get on asap if he wants it for this summer though. Really push with the SEN department.
If you start now and he's prepared to work, it may make a big difference, especially if he's losing confidence.