What a lovely problem to have!
As the state primary is currently meeting your son’s needs, I’d be inclined to keep him where he is, and use the money to enhance his (and his younger sibling’s) life experiences in other ways such as extra-curricular activities, special/educational holidays, university fund, house deposit.
I don’t have any experience of the private sector, but judging from many posts on Mumsnet, there’s no guarantee that even a good private prep school will get the balance right for your bright children. It’d be a shame to ‘waste’ the money now, when it might make a greater difference to their later lives.
In terms of supporting your children in primary, as you’re a primary school teacher you’ll know what the current school is doing well, and what questions to ask. You’ll also be able to enrich his education at home, to prepare him for the 11+.
My DD is state educated but outside school time she learns a foreign language and a musical instrument, as well as having speech/drama training. There are plenty of other ways to enhance your children’s education.
I should add that I don’t believe I’m anti-private education. I just think that, if I were in your shoes, I’d save it for later. Good luck with your decision!