That's discouraging for him to be sure. It wil not be the last time in his life he will be rejected for something..
Its part of the growth process. As a grad student he will when he completes his studies be very much busy and can choose the things that interest him in his field of choice.
For me, being happy at what one does for a living is most important and makes the work not seem like work.. Amazing how quickly decades pass by.
He is talented and gifted in several areas and he will keep those things for his lifetime. I still play my Sax and clarinet occasionally before I enjoy them both and love music. He is experiencing maybe "rejection" and it is new to him perhaps? Growing cycle...