But we also want to try KES Edgbaston (without scholarship) to keep options open and not put him under any pressure.
Just a quick note to clarify something.
You did not mean 'scholarship' but 'AP' (= assisted place).
Your DS will get a scholarship if he does brilliantly in the entrance exams and the interview (whether you like it or not, but most people like it!! or even dream of it!). The scholarship is not linked to your income and its aim is to attract very bright students into the school.
Traditionally, in order to prompt consideration for a scholarship, the child must demonstrate outstanding potential in the Maths section and their English skills would have to be reasonably good. But that was three or more years ago so things might have evolved...
please read here for more in depth understanding:
about AP: http://www.kes.org.uk/assisted-places.html
about scholarships: http://www.kes.org.uk/academic-scholarships.html
about music scholarships: http://www.kes.org.uk/music-scholarships.html
hope this helps
good luck for your DS' !!+ route