CallarMorvern
I am also puzzled by this thread.
First, the child is in Year 8 so not even at GCSE stage at all, so doing GCSE's early is way beyond what most state school children would be even thinking of at this stage.
Second, the OP is engaging tutors for maths, science etc, as far as I can understand, and one to one work in that will take the student absolutely ahead of the game, they would probably find they were ahead if they returned into state school at this point (as someone HE on this thread has found themselves).
Third, the number of GCSE's taken is now dramatically dropping as universities and colleges are demanding fewer (higher grade) ones. No longer is your bog standard comp offering 10-15 (!) as someone suggested, my dd's is offering 9 and you could drop to 8 if you want. Two of those are fillers, so really only 6 are needed, and 5 actually required to get her to A levels at the local college. Schools were bumping up their scores getting pupils to take far too many not demanding enough ones and this practice is stopping.
Fourth, you are not excluded from university by home edding. Universities take students from a variety of backgrounds- many do Access courses so have only rudimentary Maths and English- if they pass that, they can get in! (imagine that, people going to uni who didn't do 11 GCSEs). Universities make contextual offers and are open to older students as well, so if the OP's son wanted to go back a few years later having diversified (or done Chemistry and Biology A level in a year) then he would be able to do so.
Fifth, pretty much most things in education are not a one time offer. Yes, there is a standard pathway, but I have friends who did Medicine later, as a graduate, after doing another degree first, one after becoming a pilot! You are doing your children a massive disservice if you are convincing them GCSEs determine their whole future and can only be taken once at school and it's actually a lie.
Sixth, to read on here about how brilliant state school is, it appears to be a combination of an amazing community building organization and nurturing place where everyone comes out with 11 GCSEs, onto Russell Group with mental health issue dealt with immediately. State school, in the UK? CAHMS is overrun by stressed out distressed teens who don't fit the mould and are struggling to live in this ridiculous narrow world in which they are only valued and valuable through their GCSE or A Level results. That pressure on students is on US.
I have a child at a super-selective grammar, so I am not against state education at all, but neither am I for it. I worry in HE that many people go off radar. But what the OP is offering her child, tutoring/courses in core skills, plus all the rest is probably superior to what at least half the country are experiencing! Teaching in some state schools is either poor or the teachers leave all the time, my other dd is in a school like that and we tutor to make up for the deficiencies of state education!
Honestly, this thread is like falling into some type of groupthink where state education is absolutely marvellous and just perfect for sensitive autistic children ahead of their years!