Yeah, that's a tough question. I played both piano and violin when I was kid. There were times when I wanted to quit both but my parents wouldn't let me. I did eventually quit violin as a teenager but I still play piano and I'm so glad! Maybe talk to his teacher about ways to make it more fun (for example, is there a TV or movie theme song that he could learn a simplified version of, or a game to help him learn his notes better?). Hopefully he'll start to enjoy it more over time!