Thanks everyone for the replies.
Comefromaway: I'll have another look at the IGCSE option but it looked (peering into my phone on the train this morning on the mobile website) that it was the new 2020 syllabus that wasn't being recognised.
Re. doing GCSE at school but not actually as part of the GCSE class (which would be the ideal option): the music teachers know him well, and are very encouraging with performance etc, and so I'm really hoping to get some helpful feedback when I meet them in a couple of weeks. But I realise it would mean extra work for them, which they might feel is inappropriate. And it probably sets a precedent which I know schools in general are wary of. I don't supposed he's the only kid having similar problems, especially when its the arts subjects that are generally squeezed out (very academic school, hence only 2 free options when you've done all the compulsory stuff). Having read up about it, I realise it's the composition supervision that could be a major difficulty.
Titchy: yes, next port of call after the music dept will be the Latin dept! I think he'll be less motivated to study that alone but the teachers might be keener to help as he's very enthusiastic about the subject and has been getting really good marks. I think they're fully expecting him to take it as an option, so they might be motivated to try and make it possible. I know someone who knows someone who is a Latin tutor, so that's a possibility if we need outside tutoring. I know a Mum who used to teach MFLs at this school about 15 years ago and in those days she had the odd student doing an extra language, and she taught in her lunch break once a week, and set extra homework etc and it turned out fine. What the ethos is these days, I don't know.
Boys: I'd love him to drop one of the compulsory subjects but it's never going to happen. They are really strict. My major bugbear is that they all have to do triple science. My son is ok at science but it's really not his passion and I know he's not going to be arsed to work hard. He got his mid-year report today and got 8s and 9s for everything he likes, and 6s and 7s for everything else, incl. science. That speaks volumes.
Malbec: I'm fairly sure he would be capable of jumping straight in at A level (he's about to take Grade 8 on his first instrument, and he will do some theory exams when we get around to it). He's a good all-round musician, from a musical family, does a lot of ensemble playing, does a bit of composition, and he's got a very musical brain, so I think he could get on ok. But it looks like the schools with good music depts. are quite competitive to get into at A level, and over-subscribed. They say on their websites that they require A or A* at GCSE for all A level subjects studied, and on top of that there's an audition. So, if they're looking to eliminate people, my instinct is that they'll just throw out all applications from people without GCSE. His own school might be more flexible as they know him, if he decides to stay there, but I'm not entirely sure. It's something I need to check further. (Staying at his own school for sixth form has added complications, which I won't go into now - but it's why he needs to keep options open.) I will send you a DM about the online teaching - thank you!!
Doing computer science (his other favourite subject) outside of school is a definitely no-no as it looks hard and there's nobody at home to help with that! Far too hard. At least that's a firm decision.
I should just get over it, really. But it hurts a bit that kids have to make decisions so early on.