There will be lots more useful advice here, I'm sure, but DS2 (5 in August) was in a similar position to your son when he started last September.
Firstly, there is a lot of play, creative activity and topic work in Reception, so your son might find all these fun and interesting. Mine did.
There is also a huge social aspect to Reception - learning behaviour codes in school, becoming more responsible for oneself, etc - and DS2 has learned all that this year as well.
Secondly, as far as reading goes, most schools are quite happy to put able readers on books suitable for their level - DS2 has been reading Y2 books since September and is now choosing what he wants to from home/school/library.
As far as maths goes, I've not been convinced that my son has learned much this year, but I just carry on teaching him concepts, either incidentally as we notice things or by computer games etc. There is lots of real life stuff you can do with money etc. Reader Rabbit is a good series, and your son can work as many years ahead as he wants.
Do talk to your son's teacher about what he can do. I had told DS2's teacher he could read, but I think a lot of parents say that, and it took her a few weeks to discover what I meant.
She has been quite flexible about his abilities, and, though I haven't been completely satisfied he's been challenged in all areas at school, he's certainly had a great time and learned lots of things. He has recently begun to say he is bored sometimes, but I'm not sure how true that is.
Sorry for long essay, but hth.