I guess you have only withdrawn your son from school recently? There is no rush to get qualifications. He may not need them at all. If he does, he may do them when he is a bit older or a lot older. For now, why not just educate him in the way that suits him best, and put the qualifications on the back burner?
Is your son actually learning much from his GCSE study? Even for many neurotypical kids, GCSEs are a slog, not enjoyable, not very educational. There is a lot of emphasis on the assessment process and not so much on understanding. The curriculum can be quite rigid. Just about all of the home ed parents I know describe the "GCSE years" as being far less educational than anything their kids did before or later. It is the tail wagging the dog.
The school system is full of age-linked targets which for some kids (many kids? most kids?) are not in their best interests. What is so special about 16? Only that that is the end of Compulsory School Age, after which it's assumed that education may end, the child may be "lost to the system", and they'll have no future if they haven't bagged some decent GCSEs. And of course the school wants as many children as possible to get as many GCSEs as possible by the time they leave that school, so the school looks good; often there is little thought for whether being pushed through so many GCSEs is right for a particular child.
But for home educated kids, there is no natural end point to their education. There's only a technicality: you are only legally obliged to provide an education until the end of CSA. But like all other loving parents, you and I will continue to educate our kids as long as they need it, not stopping when they are 16. In the case of my child, and perhaps yours, we may be educating them well into their 20s. So there is no hurry with anything.
My 15yo has a moderate learning disability. I rather doubt GCSEs will ever be right for her. Certainly not next year. I won't rule them out altogether, because kids do develop quite a lot over time. Heck, this time last year my daughter was almost entirely unable to read, and now she can read almost everything at speed. But for now, GCSEs seem profoundly inappropriate for her. I don't want to waste her time or ruin her self esteem by pushing her through them.
Like you, I feel no urgency about getting her out the door living independently. So my focus is on helping her learn about whatever interests her, as well as on imparting some key skills I think will be useful to her: finding her way around, managing money, protecting herself from exploitation, cooking and washing her own clothes. I take her to museums, help her find documentaries, get her together with people who will talk to her about their jobs.
Formal qualifications may well be helpful for getting a job. Functional Skills will be appropriate for her someday, I think, but again there's no urgency. She seems very young compared with other teens, and it feels premature to look at how she can fit into the system. I am looking at helping her learn the skills she needs to know. Proving that knowledge to other people may be important one day, but it isn't dominating our daily life now.
"Everybody else" is doing GCSEs at around the age of 16, including most of the other home ed teens we know. But my daughter isn't everybody else.