Had the meeting with the Head. It went well.
Based on what I've learnt in this thread, I didn't ask for him to be moved up a year and focused on the end goal instead.
That the school hasn't been providing adequately for him in maths is more misinformation than malevolance. By their own admission they don't know what to do with him. I didn't dwell on what they've been doing wrong or what they could have done better. I kept the discussion on what can be done from now on.
They claim that they haven't come across someone so far advanced in maths. I find that incredible. I know DS is an intelligent chap but he's more top 1% than top 0.0001%. Anyway, they seem open, even eager, to learn how to deal with him.
I didn't have to use that line about if he were several years behind they'd be calling agencies in left, right and centre - they beat me to it!
There were other things discussed, all of which I can't remember but they did ask about and seem reassured that he did a lot of other normal boy stuff like playing football, swimming, cycling, Beavers etc.
We've reached some agreement.
Step 1: Understanding where he is now - areas of strengths/weaknesses. Involves use of external psychologist. School is happy to find one or have us suggest one
Step 2: Using that assessment to plan for his needs. An Individual Education Plan will be drawn up in consultation with us (and him).
Step 3: Working out what resources they can devote to his needs - they're happy to provide more than they are now - and which ones will be provided by us
Step 4: We'll be actively involved in the IEP and regular progress checks will be made and adjustments applied as necessary
Step 5: They will contact other schools in the area, find other children close to his ability within the "Cluster" and look at ways they can get them to work together (which beats him just being pushed up to yr5/6 to sit in on maths lessons).
Step 6: Outside school activities: We'll coordinate his outside-school maths work/clubs/competitions/anything else with the IEP and his teacher. I may end up running a club in the school if there are enough takers.
Now it's all in the implementation!
As the school is so open to learning how they can provide for him and children like him I've undertaken to put a portfolio of information together for them. It will include list of resources generally available - like nrich and murdeousmaths (neither of which they knew about), resources available specifically for schools, funding and grant opportunities, national competitions the school can sign up to (like maths in motion, thanks snorkle), expert advice on catering for children exceptionally gifted in maths etc.
It's been a good day. Your advice in here has been invaluable. I am deeply grateful.