Let him decide whether to continue the lessons. We all need activities we enjoy as a release from the tensions of life. It doesn't work to micromanage each other's hobbies. They need leisure time, and they need to feel secure and unhurried, at least some of the time. Your son's behavior seems normal.
Children grow and learn in spurts.
Periods of rapid learning are normally followed by dormant times. Children need that dormant time to rest, settle in and collect energy for another burst of learning new things. We think music education helps our kids in school with math and memorizing facts. We require them to practice every day for a minimum of thirty minutes. We sit with them the entire time and usually play along on our instruments.