@Nosuchthingastoomuchcheese congrats, your ds is very bright and possibly a genius. I agree with all the positive advice given.
I would get back in contact with the school. It must be nice for the HT to contact with such good news. Ask them with to do next, options for secondary school, 11+ and how to support him so he isn’t bored.
On maths, have a look at Uk maths challenge website. There are past papers on there if you dig around that he could have a look at. Lots of resources available websites, bond books.
Consider grammar school and private school options. Do the tours and think about it. He would be eligible for bursaries if required and you would have to fill out all the forms. It would be very financially worthwhile.
I note your point in his perfectionism which is good because he is motivated to do well and is naturally clever. Most kids seem to be lazy and clever so it’s a struggle to get them to work and concentrate. But over excessive perfectionism can be detrimental. I know one close family member who has just got to their first choice university after a struggle and only to get there and realise they are not the genius that they were in their small a level group. Just average in a top world class English university full of very, very bright people. And unsympathetic stuffy old lecturers who have seen it all before and won’t give any leeway in a pandemic. I don’t want to go off on a tangent but they have decided to come home and reconsider their options. My point is you are doing exactly the right thing by telling him to be kind and that he is a sweet boy. Being nice and empathic is better than being smart and arrogant imo.
Being sporty is a really good thing. Playing a team sport is really good because he learns to work and play in a team and also as he gets older enjoy the friendships. And also that losing is not the end of the world. Too many kids grow up and because their parents ‘fix’ the problems they find it hard when they leave home. I am not suggesting this is you at all. Just putting forward an observation.
You have a very nice problem op. Most people on this thread are desperate for tutoring advice and first hand knowledge of 11+ exam technique and the schools.