Why are you making him do this? Sounds to me like you are well beyond the point where he will EVER love this instrument.
I am also not sure that kids (even profoundly gifted ones) need a "serious extra-curricular" activity at the age of 8.
I disagree about the need for an serious extracurricular and it's something I feel strongly about. So there's that. I also think it's such a small imposition on his time. Even assuming it offers him NO benefit (which I disagree with) what's the big deal? We all have to do things we don't like. He's not unduly burdened with these tasks. True, he's only eight. But it's 3.5 hours a week. He is NOT in a pressure cooker environment. His music school is very casual, his teacher is very casual, nobody cares how well he does, I just want to see progress and a reasonable attitude. And given that the imposition on him (in terms of both time and effort) is so minimal, I can't help but think it must be due to something I have done and I'm wondering if I can "fix" it.
I should also mention that this attitude has appeared in other areas, albeit to a lesser degree. It's not a cause for concern, but I do feel like he tends to shirk responsibility a little. Not to a degree that's inappropraite for an 8 y.o., but the attitude isn't strictly limited to the piano. For example, if I ask hime to help me out with sock matching I always get a little push back. And he probably averages less than one chore a day, so it's not like my expectations are unreasonable.
In the long run, I am not averse to him quitting. But I have two reasons to not allow it at this time:
1. I have no way to replace the piano with another
high quality extracurricular at this time. Maybe in September. I like the suggestion of allowing him to switch to another instrument.
2. I feel like his attitude has devolved into something unreasonable and I don't want to indulge it. I would like to see an improvement in his attitude towards task he percieves as unpleasant before I consider letting him quit piano.