I wouldn't worry too much at school.
And now the exams are much more about explaining your working, which a child who is naturally mathematical can struggle with (as it's so obvious to them), so peer helping may be beneficial.
I've 3 dc who are mathematical (as me and dh are). My oldest did use to choose to do maths workbooks, the others haven't really. I don't think that makes as much difference as some of the (often silly) conversations we've had at odd moments along the lines of "infinity being a concept".
If he wants maths books, then get them, if he doesn't want them then don't worry.
The best way to stretch is often letting them investigate. Let them spot patterns in say 11 time table (go beyond 10) or work out how they'd count the number of insects in the garden type thing. See if you can find a book with that sort of suggestion.
I would be careful about things your relatives say. He may be absolutely brilliant, but if he starts hearing that he may start reacting accordingly, and that can effect him.
Dh has a relative that is always trying to push how brilliant their dc is. They phone up to tell us that she's just done something they consider amazing. It seems to be a accepted fact among some of his family that everything has to relate back to that this dc is brilliant.
Result of this is that the child in question thinks they are brilliant. They're quite capable (and did last year) on hearing that another relative has A* in further maths A-level and saying "that's nothing, I'll do better". Their identity is totally wrapped up in that they are brilliant.
My dc are way older, and I've helped with maths groups in primary schools of various ages. They haven't yet told me anything that the dc has done that I've thought is anything unusual for the top 10%-25% in a normal class. I've had more than once where they've told me something where I have known that my dc's class did it over a year earlier.
This is going to hit them hard when they finds others that are better because at present they think that the world is in awe.