I'm not sure what I want him to get out of it. Fun, joy, challenge. But, I'd sacrifice those things to reduce my stress, I think.
My recommendation would be to think long and hard about this question, then, because the correct course of action stems from this answer.
In our household, DD7 was enrolled in guitar lessons as a result of task-avoidant perfectionism issues. She needed to work on something that's hard, so she can see how mistakes are normal, part of the learning process, and practice is necessary in order to acquire a skill. When practice became an issue, we let her drop guitar, because she was already getting the same value from other sources (soccer, gymnastics, dance), and the lesson is taking hold.
The door is not closed on guitar, though. Daddy got his first electric for Christmas, and DD is intrigued. I reiterated that she can have one of her own if she earns it by working on the one she has.
You indicated that piano is something he has been intrinsically driven to do, so it might be a good strategy to stop it completely. The suggestion to pay the teacher to keep his slot open is a pretty good one. You could explain that his teacher requires him to practice, but arguing with him about practice is driving you insane, so you're just not going to argue with him anymore, he's just not going to do any more piano lessons for X months. At the end of that time period, he can go back to lessons, but daily practice is not going to be a major source of arguments between you, or you'll pull the plug again.
