Lawnmumma:
With a DD1 in Y6 and soon off to secondary and DD2 in Y4, can I sincerely say that school isn't just about how bright you are but is also a social experience.
You strike me as someone who's got some very clear ideas on how you want your child to progress but I'm slightly surprised at your desire to rush through these early years - they truly are precious. There will come a point when your DC just wants to read to themselves, doesn't want you to give them a kiss or a hug in the school grounds and kind of drops off kissing you goodnight. It's all natural of course, they're growing up and that's a good thing, but it is bittersweet. (I'm at this twilight stage with DD1 - who every now and then forgets and gives me a lovely hug in the playground before trotting off to class, but they're getting rarer).
My genuine advice is let your DC have another year at nursery where he builds his social skills and confidence prior to starting school.
Being the youngest in a new setting is always tricky - regardless of how bright you are. Being slightly older in the year group has its advantages - and as you clearly have a maths background you must surely be persuaded by the statistics that show that autumn born children consistently achieve better results academically over summer born children. www.nfer.ac.uk/publications/33309/33309.pdf - and don't just stop at the first summary of a study (Fogleman & Gorbach result that length of schooling explains differences in achievement). The overwhelming consensus seems to be that there is an advantage to being slightly older (?more mature) in a cohort. (see also www.gov.uk/government/uploads/system/uploads/attachment_data/file/182664/DFE-RR017.pdf).
Now in terms of your second worry - once your DC is at primary will they adequately support him. I think you have to be highly aware of your rights and prepared to politic for those rights to be recognised. I don't advise going to battle, but do be prepared to wage a campaign - possibly quite a long one.
Many schools are resistant to register a child G&T. There is a G&T feed here on MN education and I suspect you probably should start visiting once your DC starts primary school. As far as I can work out registering a child for G&T academically means the school must adequately provide for that. This is a good starting point (although somewhat out of date perhaps): dera.ioe.ac.uk/2515/1/982_primary_provision.pdf
I think my suggestion would be to be prepared to support your DCs interests yourself should the school not be willing/ able to do so themselves. One solution you do have if your child is far ahead of his peers is to request that the school allow your child to dip into lessons in a higher year group - possibly joining maths/ reading with a higher year group. Some schools combine year groups anyway and teach at ability/ other schools combine children from different forms of the same year group and then stream for ability - regardless what you should be pressing for is work that is adequately differentiated to meet your child's abilities.
However - do accept that the school has all sorts of priorities and is rarely rewarded for helping high achievers. The natural pressure is to focus on those failing to achieve their targets or close to exceeding their targets - because this benefits the school in terms of how their performance is assessed. So in dealing with the school and requesting support for your G&T child, remember that in terms of priorities for the school, although I'm certain in an ideal world they'd love to do more, this will naturally be less important/ crucial to them in terms of the criteria they are assessed by.
HTH