I can understand why you want him to continue in piano after so many years and he's good at it. On the other hand, is it worth making both of you miserable over this? At some point, he will likely stop piano anyway since very few people become professional pianists.
Is it possible for him to start a new instrument and be in a student orchestra or band? He obviously can read music well, and this could be a new opportunity for him. You could have him pick the musical instrument.