I have read almost all the other thread but I'm another one with deep concerns about home education.
A close relative home educated for similar reasons to you (he didn't like school so assumed his DC wouldn't and it was a self-fulfilling prophecy.) His DC didn't want to learn some subjects so they didn't get taught so then his DC decided they were bad at those subjects.
The DC is now in school and they have been sympathetic about the poor level of progress before this.
As others have pointed out, it sounds like your DS doesn't do the subjects that don't interest him, either. Part of learning to learn is learning how to work on subjects that don't interest you much, but that are useful.
My own DS has been excluded for days at a time and has social communication difficulties (not ASD, at least we don't think so, but similar). Being at home was incredibly isolating for him, and has not helped his mental health one bit. And that was just a week or so.
My DS needs to have very low level exposure to typically developing children on a daily basis as there is no other way for him to learn to socialise with TD children. They need to be the same children every day as he (like many children with social communication difficulties) takes much longer to get used to communicating with any one peer.
We spend a reasonable amount of time with other adults and children of different ages after school and at the weekend (e.g. his regular babysitter, Cubs, family members, children he knows slightly at the park, birthday parties, round at friends' houses). This would not substitute in any way for daily exposure to other children at school. If he was at home full time we wouldn't do any more of this, either.
I too am concerned about your DS small range of exam subjects and the casual attitude that "they can do exams later". I don't think having several years "in limbo" would have done my young relative any good at all. They would have (rather like my own DS who is a lot younger) become more and more withdrawn while waiting to sit their exams. I know adults who have done access courses but usually after either another career (e.g. switching to Engineering from English) or after a productive few years working in something that didn't require qualifications. Not alongside waiting at home to go to university. If he doesn't get the grades or suitable qualifications for university entrance at 18, will your DS be working in retail/a care home/a call centre for a couple of years while doing an Access Course? I know my young relative wouldn't be able to cope with that kind of job.
And yes, universities will want all the subjects to be sat at the same time. It's not stamp collecting...
You are lucky to have so many Home Ed groups where you are, and I know you say you "know families who are less well off who home educate" but even if it was not for the lower earning power if you can't work/you work part time, it's expensive to take your children even via public transport to multiple groups.
We have nothing in our area (unlike a PP, we have some groups for self-motivated teens, but it's art maybe once a month at someone's house) for younger children so it would be irrelevant to us anyway.