It's your problem. You have neglected him and always yelling at him. You have to stop acting like a child, I feel sorry for your son. You are the one who is losing control and as a result, he is losing control himself because he is unsure of himself, and has no confidence. For god's sake, he is only a child, not even 7. I feel like you are crushing his ego. You need to step back and take a deep breath. Who cares about piano, sometimes you have to let the small things go, the most important thing is to make him feel happy and good about himself.