So what is he doing with all of his time, if he isn't doing schoolwork, chores, or practicing his instrument? If this is about control, then what does he actually have overt control over (as in, to the point of being allowed to take failure as a natural consequence, or lose access to a desired task, object, or experience as a natural consequence)?

If he is genuinely interested in something, yet is unwilling/able to do it, that also suggests looking at depression. Or could there be any aspect of perfectionism/performance anxiety involved, for instance, where he avoids doing things he likes because early attempts don't match his internal expectations? Or that he self-sabotages, because it feels more acceptable to fail intentionally (control) than to fail when he was really trying (out of control). I know you've spent time working with him on growth mindset, but sometimes environmental feedback speaks louder, and the objective outcomes of his school failures these past years have been moving backward in instructional level, and a great deal of psychic distress for both him and you. Recognizing how one learns from these difficult years is usually possible only in hindsight, with a longer-term perspective--a challenge certainly for any young person, and, really, for everyone still in the thick of the situation.

Thinking of you.
...pronounced like the long vowel and first letter of the alphabet...