The problem with high IQ kids with ADHD is that at the beginning of their education things are easy and they are flying. My DC was reading fluently before starting R, was, like yours years ahead in maths, etc.
I always knew on some level that they probably had different attention span, but it was not an issue academically in primary - and teachers never suggested testing, saying it’s not needed if I asked.
Things do change in secondary, especially in the years approaching GCSE curriculum. The amount of work increases, and so is the breadth and depth of the various subjects.
And this is where kids like that run into trouble - as previously the IQ let them fly without needing to develop learning strategies for their different attention.
I had friends with older kids and I saw what can happen if a diagnosis is delayed and kids don’t get right support. GSCE/A-level exams are hard - and no reason not to make sure your kid has accommodations, if this is what is requited to make sure they have a level playing field.
I am just saying all of this to keep in mind, even if secondary seems far away for you. It’ll be here sooner than you think. Assessments take time, especially if you need to go via school/GP. I’d find out how long it takes where you are and start in time to get it in place by Year 9, so that your son has the right support and accommodations in his GCSE years.
As to extending your son’s maths - there are lots of materials. I’d make sure to give him a lot of world problems - not just get him to learn more and harder concepts.
In addition to nrich, have a look at these books - Maths Plus Word Problems - they have great extension problems.
For practicing maths skills - we have always used Schofield and Sims books - took them
with us on vacations for extra practice before 11+, for eg.
Also - have a look at Primary Maths Challenge website. It has past papers and materials. He is probably a bit young for this - but you’ll see that being good at maths often means not just being able to do, say, differentials at a young age - but being able to solve complex problems applying often fairly simple maths.
https://www.primarymathschallenge.org.uk/
In addition - have a look in your area -
where we live there are weekend maths clubs and those could be fun for a right kind of kid.