Menagerie, your son sounds lovely and very bright and motivated.
Our DC's school sounds much like yours. It gets very good results and my DC has good friends there. But all the kids are tutored from at least year 3 or 4 (and a good chunk of them much before that, with Kumon and the like). I realized this year that I was going to have to do something to fill in the serious gaps for DC1. I've used the CGP books, appropriate to school year - one page of literacy, one page of maths, each morning (you have to find the time that suits your DC best), or at least most mornings, depending on how tired (and, therefore, counterproductive) he is. It usually takes about 20 mins altogether - sometimes a bit more, sometimes a bit less, but nothing too strenuous or time consuming. We, he and I, chose mornings because he is more awake and then he can just play after school.
Personally, I have struggled going back to basics; partly because I was never very good at maths and partly because I forgot (and was, consequently, frustrated) that some of this basic stuff has to be learned and isn't just automatic. But I would also say it's made me closer to my DS, partly because I'm moved by his effort and striving; and partly because sometimes he can laugh and explain things to his stressed mother
If he's got a concerned, caring mum like you, he'll be fine, whatever path he goes down.