Hi - retired TA (male) here :
I don't have much to contribute, other than to say that in my experience most schools do have great difficulty in accommodating pupils with skills that exceed the average for any given age group. A lot of resources go into SEN and trying to raise the standards for weaker pupils, but although the G&T label (gifted and talented) is bandied about, I don't know that many children actually get special treatment as a result.
If DS is working with groups two years ahead of his peers, then you are fortunate that the teachers have at least responded to his needs to that extent.
Depending on the size of the school, it may be that there is no real Maths specialist available, and the class teacher knows just enough to teach to the required Year 6 average ability, and not much more.
It must be embarrassing for a teacher to realise that a pupil knows more about a topic than they do, and this used to happen particularly in ICT years ago, (say 20 years) when the average teacher was terrified of computers, which were quite new in schools then. My own DS began learning to program a primitive computer at the age of 6. By the time he was in Yr 6 he was starting up the school network each morning, and helped the HT learn a new word processor program (because that was what we used at home.) But when he went to grammar school what they were doing in ICT then was baby stuff compared to what he could have done.
If you can contact a maths specialist at a local secondary school, they might be able to advise you of avenues to boost Maths experience, and some secondary schools do run special Maths weekend workshops for people we might rudely call 'boffins'. Quite possibly there might be info on the web from Johnny Ball, Carol Vorderman or Rachel Riley and suchlike.
As well as being able to DO advanced Maths exercises, I think it is also essential that the whole process is fully UNDERSTOOD by a pupil, and that he isn't merely performing to a learnt (but not understood) formula.