I think intelligent parents would wonder why, when the average child is normally capable of 5 passes at 3-4 in GCSEs, their child is not.
They would long, long ago visited the school to ascertain if there is an issue at school, whether their child was lazy/disruptive and if not, they would have sought an educational psych assessment to rule out dyslexia, dyscalculia and even ND issues - which I see you are awaiting.
If I were these parents I would be urgently exploring the diagnosis so that you at least have it in place for 6th form. Get the school, GP, everyone on side and get it expedited. Tbh, I’d speak to the GP about who they’ve referred to and explore a private referral to the same clinic as that can be done within weeks. I’d take out a loan/get a new credit card, sell stuff on vinted, borrow from GPs to get the money together to do this.
However, I’d also advise you not to panic about the qualifications themselves. A friends’s DS failed all his GCSEs for reasons of lack of diagnosis and correct support at school - but fortunately got a place at a local 6th form where he was able to start afresh. Y1 he redid 5 GCSEs and in Y2-3 is now doing BTECs and an A level with the view to getting onto an apprenticeship degree after that. My own DD did well in GCSEs but dropped out of 6th form twice. Eventually went back to tech college to do an Access Course and is now at university.
There are lots of alternative routes to higher education and training schemes, but your DS won’t be able to access those unless you get to the bottom of WHY he is failing - and prioritise getting this sorted now. Your own level of intelligence is not the issue but your resolve in advocating for your DS is.