GCSE music is much different than theory or intrumnetal qualifications.
Dending on board:
30% performing [in a group and as a soloist -can get good marks being around grade 2-4]
30% composition [ usually the hardest bit as most people who learn intrumnets haven;t writen anything original themselves before]
40% listening and writing exam. [listening to music and answering questions about it, usually with lots of terminology and 'aural' skills.
The music GCSE, I personally think, is not rated as an 'easy' option but it is possible to do well in it and not read music that well. The coursework [performing and composing] is a lot of work load if student has had little experience of performing and composing. Easier if student has loads of peices learnt and up thir sleeves etc.
There are many subjects you can take in 6th form/a level with having taken them at GCSE. Depends on the sixth form. Compentency in numeracy and literacy [GCSE in english and maths] usually important to be able to take A levels.
For example if your son didn;t study history/geography etc, chances are he could do these at A level..... Ask local colleges?
Again, Grade 8s are worth UCAS points SEE THIS but most degrees ask for music A level. Probably becasue the Alevel contains more skills in writing music, history of music etc.
Most importantly, cos I'm getting far too serious, if he enjoys it, go for it. Music is such a rich part of human life and can lead to maaaaaaaany maaaaaaaaany differnet things. Even if he just learns stuff at GCSE that helps him enjoy music in his life.