Your situation is somewhat different from mine, as your ds's school does seem to be recognising his ability in maths. However, I thought if I briefly go over what happened with dd, it may give you an insight into possible options.
Every year since year 1 I would have conversations with teachers where I said that dd was advanced at maths and they said that though in the top set she was not that far ahead and in some cases middle to bottom of the set.
By year 4 dd realised she was never going to learn anything new in maths at school and so in Y5 we decided to home educate until the end of primary.
We did not want to spend the whole of secondary arguing with teachers over what level she was and so she sat her GCSE at the end of year 6 and got A*.
Her new school is absolutely brilliant. She will sit A level maths early, but they also get her to do sideways education: UKMT and the like.
After A level we will look at "Futurelearn " and other MOOCS so that she keeps on progressing in maths until the end of secondary.
I would say it is important to be asking questions of prospective secondary schools and try and sound out their attitudes and ideas for your ds's education in order to find one that will suit you son.
If your ds likes doing UKMT type questions, ask the present school if he can sit the JMC next November. (It would mean them finding a secondary in the area that sits it). Have a look on their website anyway, as it has a lot of puzzle type questions for maths.
Khan Academy and nrich could also be worth a look at.