Sorry to hear things are not going quite well for you and your DS at the moment @amr78 I can totally understand how it feels, even NT boys mature much later, and autism can completely mess up executive function for years when big decisions have to be made. My DS doesn't have ADHD, I can only imagine how hard it may be with combined challenges.
I'd say maybe try and think about longer term perspective, not just immediate concerns of GCSE/A Levels/uni/music. My DS had stellar GCSE, decent A Level results and studied Maths at a Russell Group uni - he still massively struggled to find employment. It's extremely, sometimes unbearably stressful for autistic people to do job interviews, especially if they are prone to "selective" mutism (there is nothing selective about it, of course). So I would shift thinking perspective from "how good his exams can be" to "how to secure housing and basic income stream for him". It may be relevant even for high functioning autistic people as they can go into burnouts from basic social functions required in adult life.
There is a big jump between GCSE and A levels, GCSE were a walk in the park for my DS and he genuinely could not understand what the fuss was about as they were incredibly easy for him. A Levels were a shock to the system as they require different study skills and much better executive functioning, so it's not about the content or workload, rather about planning and organising your studies and following your plans. It's one of the weakest points for many people with ASD. Your DS's refusing to practise may be due to both executive function impairment AND autistic burnout during exam years. I think you might need to decide what to focus on at any given time i.e. if you want exam results, you might need to pause music; if you want professional music, you might need to postpone A Levels.
Personally, I think the professional music path is very tricky so I'd rather put music on hold, but everyone's situation is different. There are exceptionally talented musicians that don't need any exams and there are parents with connections in the industry. I'm merely speaking from an autistic person's perspective - it may be very hard to cope with both exams and high level music practice.
It's a tricky situation for parents too, as DC are considered independent adults by unis and other organisations and they are supposed to figure out everything for themselves. For people with ASD, depending on the diagnostic profile, it may never happen or at least come at a much later point in life than usual. I wouldn't expect them to "understand the consequences", not in teenage years at least.