I think you’re being unrealistic about what the school can or should be doing, but also if you used to work there perhaps that is because you know they aren’t very good. That would then raise the question of why you would send him to a school you don’t think is very good.
You’re working with the benefit of hindsight - in year 10 he was on track and in the November mocks he will have been in the normal situation of being a grade off his realistic target grade and two away from the optimistic target (a 7 is an A in old money, which is optimistic for a child with a scaled SAT score of 103 i.e. ‘average’ and therefore grades 4-6 would be typical). Plus, you knew at that point where he was and you weren’t overly worried, so why should they be? The most recent tests have then been absolutely shocking - to you and to them - and they have reported them to you now in a timely fashion. In my school, he would be flagged as underachieving and we would be discussing that and I think parents are contacted. However, there is a limit to what we can do - he is one of, at a very conservative estimate, 200 children that each teacher sees each week. In his class, he’s one of 30 children who all deserve and need the teacher’s time and attention. Teachers can’t learn the content for him and they can’t practice exam technique for him. It is also important at some point for kids to take responsibility for their own learning - this is that point. If you (and teachers) do all the work for him now, it’ll only come unstuck at A Level instead. Remember, your experience is working with primary school children while he’s got a just a couple of years left before he’s an adult. Secondary school is about gradually removing the scaffolding and support so they stand on their own two feet while there are still people around to catch them when they fall. What does he say about his current grades and why they happened and what he’s going to do about it?
In reality, you have been given an enormous amount of feedback on the free education that your child receives (you pay for the co-curriculum, so it’s a total red-herring to say this is a fee-paying school). Even in a private school, it’s still at a point where the school can do very little to figure out what went wrong. It’s up to you to figure out what has happened between November and February that has affected your son’s results so dramatically, and then help your son fix it. He has got decent exam technique - he used it to get 5s in November - so the problem is that he doesn’t know enough now and is then getting into a panic/spending far too long on some questions in the exam.
The only possible valid criticism you have is if timing really has been a genuine issue for years and they have not considered exam access arrangements before now. That wouldn’t mean he would get them - there has been a significant tightening up of regulations on this once almost a quarter of students were getting extra time. And, even if he should have been awarded it, that still won’t account for a 4 grade drop, so not the pertinent issue here.
In my subject, to get a score as low as a 1, 2 or 3, you have to not understand the content. It’s not about running out of time. To be frank, most kids who run out of time, it’s also because they don’t know the content - unless you have a diagnosed issue that affects processing e.g. dyslexia. He’d not have got GDS on the reading paper at SATs if he was the level of processing challenge that results in a grade 2, nor would he have got a 5 in previous mocks. If you got 70% of the marks on 70% of the paper - i.e. you knew your stuff but you couldn’t process it quickly enough so left the last few questions - you’d still get a 5. That doesn’t seem to fit what’s happening here.
I have enormous sympathy for kids facing the current GCSEs as the content to learn is vast. Is it possible that previous exam results were not so bad because they were covering only recent work completed but once the mocks were covering the entire course, including work done well over a year ago, then his revision strategies just didn’t cut it (he had been relying on relatively short-term storage of recent content?) Get copies/scans/photos of the mock papers and go from there.