Thank you all for your advice!
At home, I have no issue as such. Ds is a lovely boy who is spending quite a bit of time playing football and tennis, playing with his brother and then doing stuff that are at his level of abilities (playing chess with DP, I take care of the 'literacy' pov as I am comfortable with it. Lots of reading, explanations, talking about books, paintings etc...)
The issue for me isn't about what he is learning (ie the content as such). As adventure said, they learn quickly and can forget just as quickly (as I learned myself as a child).
I am more concerned about what he is learning about learning ie whether it requires concentration and effort or not. Whether he will be able to learn how to handle stumbling blocks (if he never encounter one until he is well into secondary or even after, it will come as a shock and I am actually not sure he will be able to get over that hurdle).
Also the fact that he is disengaging with the teaching at school with all the problems it can create.
So we could do some extension work at home. Both myself and DP have no problem with maths and science so doing some stuff with him isn't an issue. Even in secondary school, I am sure I could 'extend' him at home if need be. But is that suitable to do so?
If we teach him percentages and fractions (2 things he asked about), he will be able to learn and understand them. But then the situation (I am bored, it's too easy) will get even worse at school. Which is something I want to avoid.
I know some gifted children will enjoy school just for the sake of being in a social environment. Ds doesn't because he both one of the oldest in his class and very mature emotionally so he is struggling to really make friends. He is well integrated in his class, getting on with everyone but in his own words, he doesn't have a 'friend' as such. So the 'teaching' environment has quite important for him.