Try Ravensburger Labyrinth - Moving Maze board game - but you'd have to play with him.
There's a strategy game called Hive that is surprisingly simple but interesting. Hexagonal pieces.
There is always learning to draw - this can be quite a good absorbing and challenging solo activity - learning to draw animals (or whatever he is interested in) from a book.
Have a look for tangram puzzles - again very simple but some can be challenging.
Look up a pencil and paper game called sprouts - very simple - he might find that interesting - or not as the case may be - you just try as many different things as you can and see what he connects with. There is battleships - again paper and pencil. Classic board game like Go and even Mahjong is you can find a cheap set - I used to play all of these when I was a kid.
Maybe try to get him to learn a musical instrument - do you have access to a keyboard or could you borrow one for a bit? Sometimes primary schools have a largely underused budget to invest in musical instruments or peripatetic music teachers - ask if they do or would be prepared to try and support your son learning something like piano. I've found that genuinely talented kids generally have more than one talent - so don't stop wondering or looking once you think you have discovered one area where they seem to lie outside the norm - greater talents may lie elsewhere.
There's nothing wrong with going deeper with his maths but it's worth exploring other areas that might challenge him. It's hard work sometimes keeping a bright child engaged - you really have to listen to your child, watch them and pick up on what type of activities grip them and then think hard, research and dream up new things to introduce him to. His happiness and sense of worth - not just from excelling is really important - so you praise effort not achievement and recognise and appreciate his many other qualities - whatever that might be - that he is sensible, fun, imaginative, a good listener, a good friend, fair, helpful, kind, forgiving, trustworthy, generous, etc.
If I have time I might pm you and say a bit more - things I'd prefer to say privately.