Acknowledge his frustration.

At this very moment, my high schooler is playing piano as I am tying, simply because he wants to. He stopped taking piano lessons many years ago, but the early training sure cultivated his musicality.  Sometimes, he plays because he is sad, sometimes because he is stuck in his quest for something, other times, because he does not want to face his immediate tasks.  Playing piano and outdoor exercise are not mutually exclusive. The fact that your son feels frustrated, it is a good thing, he realizes that there is room for improvement. This is not the time to ask him to quit, it is time to help him figure out how to improve, including stepping away momentarily from it.