I have to say that my nephew's view (in hindsight) is that Music A level was a big, big mistake. He was predicted to easily get an A in Music but only got a B. The problem is the coursework content and the odd way that some of this material is marked. It nearly cost him his place at medical School. If you look at the grades for A Level Music in many of the top schools you will often find that they are very disappointing - even if the children are exceptional musicians.
I've heard this elsewhere KenR - but it may depend a lot on the board - my son was part of a group of mostly Grade 8 + musicians at a KE grammar who did all do well at A level, including the performance, but they were with OCR who have a real-life visiting examiner. He really enjoyed it , especially as a break from the Maths/ Further Maths and made some very good friends in the group.
Presumably the OPs son has already been offered the potential choral scholarship on the basis of his current musical skills - the point now is to ensure the best chance of getting an offer (which will depend on his personal statement/ GCSEs, BMAT, interview, relevant work experience etc etc), and if the offer is forthcoming concentrate on the best subjects for the vital AAA ...