DDgirl I have to say up front that I think you need a maths tutor. I of course don't know your personal circumstances, but if you can afford it, get one.
I'm working on thin data, but I don't think your son has access to what I'd call a maths teacher at all.
Merely being a Ruth Kelly, should not make a year 4 get past his teacher, not a Richard Feynman either, maybe a Carl Gauss, but there was exactly one of him.
Would you expect any year 4 to beat his games teacher at arm wrestling ? Many maths "teachers" are the equivalent of paraplegics who hate sport.
Depressingly few maths teachers are properly qualified, I suspect you've hit that.
That being said, I'm not sure the average maths tutor is what you need. They typically are used by better off parents to patch over limitations in their kids. Nothing wrong with that, but fixing is a different skill from building.
I'm a pushy parent, and I'd be pressing other buttons by now.
First and obvious button is the black and yellow one with "eject" written upon it.
Your state school isn't serving your child. As someone who's been there, I sympathise, but you can't fix the system.
You can run away.
Taking DDs statements at face value, her DS is a prime candidate for a bursary to a private school. They are more likely to have a real mathematician. Many teachers of maths aren't qualified to deal with average kids, much less stars.
Of course it should be a fighting retreat, but I don't think you are going to get what you want, but I believe you can make things less bad.
Next button is the nearest university library. As a taxpayer you have the right to read books there. Yes you do. I can dig up the bits of paper that will get that done, if necessary.
If you are in London, I can explain how to get a (relatively) cheap borrowing ticket from university libraries.
As a pushy parent, I'd be on the phone to some real, actual mathematicians. Explain the situation, and asking them to help you understand if your child is as good as you think. Again with all due respect, you can't possibly know.
Many top grade people didn't think their support at school was great, some teachers seem actively hostile; and this leads many to what I will characterise as a "protective" attitude to young talent. Some will politely tell you that they can't help, but a few calls will get you in front of a good guy.
Personally, once I had expert (ie outside the school system) advice that my kid really was a star, I'd ask the tame maths prof to recommend a PhD student to tutor. They're not expensive I'd guess 10-15 per hour.
You might ask why the state doesn't do that...
Would be pathetically cheap, but arts and sports have a higher priority.
Next buttons are right in front of you. There are huge amounts of maths on the web. As roisin rightly says most of the best mathematicians are self starters.
Wikipedia is a good start, and in early days you will be able to track it yourself.