First of all, congratulations to your ds. You must be very proud that he dealt so well with the scholarship test.
A few things worth thinking about, though.
If you're a SAHM, I would have thought it would be pretty easy to arrange musical, sporting and language-learning opportunities outside of school (and/or get music tuition within your child's state school via a peripatetic teacher or the County Music Service if it's any good). If you want it all in one package within one school (even if, in reality, the quality is lower than it would be if you sourced these things individually yourself and you no longer have spare cash to make up for the private school's relative weaknesses), then private school is obviously more what you want.
If it's a prep school, then does it go on to age 13? Or is it up to age 11 only, hence sending a lot of children to the (presumably) state grammar school? Can you afford to remain in the private sector for secondary level, as that's what a prep school is really supposed to be preparing you for?
Tbh, imo, there is nothing a child is expected to learn in primary school that, for a bright child, requires phenomenally good, specialist teaching. Surely, it's either unnecessary or is setting your child up to be bored at state secondary level, because he's learnt a lot of work in, eg, history, or geography, physics, chemistry, etc, that will only be repeated?
Finally, do bear in mind that KS1 and KS2 are very different not only in the private sector but also in the state sector. Do you know what Year 3 is like in your ds's current school? It doesn't seem right to compare year 2 of your state school with year 2 or 3 of the private school - surely better to compare what your child will be moving on to in both instances?