tbh OP I'm not sure what you can do.
The comprehensive system system in the UK often struggles to provide an optimal environment for the most able children because there is simply not a critical mass of them to cater for.
Mixed ability classes compound this.
If you do not have selective schools near you, or a school with a much higher % of high ability children, or are unable to afford an independent selective school, then your options are limited.
However, I would, for the time being, remain positive about school (obviously at some point you will both have a full an frank discussion about it - but not yet). Point out all the things that are new/different. Keep bigging up the social side of school, which is very important, and point out that even in the most academically selective schools, the kids are just kids, enjoying football and parties and Play Station and Friends. They're not all wandering around discussing physics
.
And finally, I would take your son's education into your own hands. My view has always been that I educate my DC and that school is just one resource I use. There are lots of evenings, weekends and holidays in which to provide challenging activities. Indeed, your DS will begin to winkle these out himself as he gets older.
Chin up.