Winchester, Sevenoaks, Wellington for 6th Form?

[bohomama]

My DS, now in year 10 in a British school overseas, wants to do his A Levels in UK. Not a bad idea as DH's expat contract is not secured for the next three years. DS is very academic. Does minimal effort and comes out in top sets. Sore point is that he is not thriving at his school despite good grades.
We are looking for boarding schools that will:

1. stimulate him
2. have a big enough intake in year 12 that he won't feel like an outsider

He is musical but not particularly sporty. We have not decided on IB vs A L, nor SS vs Coed. Current schools we are considering are: Winchester Sevenoaks Wellington Any advice would be hugely appreciated particularly as Winchester does not have much info on website regarding the testing process for year 12.

We are abroad at the mo too. Hear great things about Uppingham and Oundle. Both good musically and good for boarding. There's also Lancing but less academic there I think.