I am currently teaching DS7 piano. I made sure that the practice is light (about 5-10 mins a day). I also made the pace pretty slow because he has many things to do after school. DS gets used to the idea that he needs to practice everyday.
I think your DS just need a break. Keep in mind he is not going to lose what he learn in piano. One day, he may want to learn some other instruments and he will bring this knowledge with him. He will advance very fast.
Or maybe he just want to compose one day, or even learn to play by ear. He will still need the knowledge he learned from piano.
I had piano lessons for 12 1/2 years. At the end, I was so tired of practicing. After I stopped the lessons, I learned to make up chord and played by ear, which I had never learned from my piano teacher. I enjoy the randomness of putting notes together to make a piece of good sounding music.
He will take what he learned from piano one day, and he will learn something different and make it better. I don't think you should be sad. Quitting doesn't mean he is quitting for life. It only means he wants a little break or to change direction.
