Why are you making him do this? Sounds to me like you are well beyond the point where he will EVER love this instrument. Continuing so his brother will continue does not seem like a good reason to me.

My gifted child is 15 now. One of the things I have learned over the years is that she has many talents. But if she hates an activity, talent does not really matter. She HAS tried many things over the years, and has only settled on her most serious interests in the past two years. Piano was among her activities in her early years, too. We have encouraged her to continue to do something musical, but she has settled on choir/voice instead of piano (although she still plays for fun sometimes, which she never did while taking lessons). So that is another option -- if you are very keen on music, allow him to maybe switch to some other instument or voice.

I am also not sure that kids (even profoundly gifted ones) need a "serious extra-curricular" activity at the age of 8. I try to expose my daughter to a variety of activities. As long as she finishes a season/year in something, I have allowed her to switch if she desires. The one exception is if we have to buy expensive equipment. Then I have asked her before the purchase to commit to two seasons or years to make the investment more worthwhile.