My two are younger so I have the joys of this to come! I noticed that the OP's DS is specialist at a music school though; my nephew,now 20 was too, at Cheethams, and my SIL went through a similar thing in his teens. She was tearing her hair out at times, feeling that her son had this wonderful opportunity (paid for by government grant too!) and that every now and then he was in danger of chucking it away - missing homework and practice, eating junk, not getting enough sleep etc. She came to the conclusion that it was sensible to make a few rules and stick to them consistently, but let other things go, which is pretty much the advice on here. She also said that when you have a kid who is in this type of fairly unusual situation - training for a specialist career - then there are pressures that most kids don't have. Her son had to follow a very structured routine a lot of the time, he had to maintain a level of focus and commitment; his education was amazing in some ways but also a sacrifice - he could not study a lot of subjects because of the big focus on music, and was basically from a young age having to put all his energies into his music which is lovely and will earn him a living but realistically he will never be well off. MY SIL said she knew she just had to cut him a bit of slack.A lot of kids in this type of pressured situation go under - he knew girls at his school with eating disorders, a lot of stress etc. The boys seemed to fare better, which is interesting isnt it? - maybe the more laid back attitude has something going for it! Her DS wasn't the hardest worker, but he did well enough in exams and is now at music college, and having to manage his own life/practice/work balance, so its maybe a good thing that SIL backed off as he got older and expected him to take more responsibility.
Apart from the specific pressures in this case, I'm sure a lot of this is very normal teenage stuff - testing the boundaries, wanting to express his opinions etc. My own kids are still at the stage where I can control homework, meals, going out etc so I know I have it easy right now, but from friends with older kids I can see how hard it gets. I also think you have to take a longer term viewpoint sometimes. When your DS is an adult, you will be glad that he has formed his own opinions, and learned (maybe the hard way, but hopefully without too much fall-out) that HE is ultimately responsible for his life, how he lives it etc. Nothing worse than an adult who hasnt learned to do this and become their own person. I'm sure it will all work out, nearly all teenagers, however horrid, turn out fine!