Musicians who say that are wannabes. I guarantee you those musicians urging 'caution' are jealous of his talent and heart.
I'm not 'known', but I'm an artist and I enjoy my art. It expresses me. My love of art started when one family member made positive remarks about something I drew. It stuck with me all of my life, and I never did quit getting positive feedback from others. But not from everyone.
One thing I have learned along the way is that other artists often do not share in the sheer joy of anyone else's art, unless they consider themselves far beneath that person. Most act like what they do is out of pure talent, when sure talent does matter, but drive and persistence and practice matters the most. Those are the jealous artists that will not teach their techniques or the finer nuances they learned. It's a rare artist that truly mentors or will pass the wand, so to speak.
Beware of the naysayers. Don't let them influence your son or you. Let him change his mind all on his own, if he is going to. If in his heart he is really the teacher, not the star, he will see that on his own and it will happen naturally. Some become instrument craftspersons from the love of the music. I bet he is really intelligent. Bipolars usually are.
Take a look at Susan Boyle with her presumed handicaps. Who knew? (and I wish I knew enough about musicians to name them). Yet some utterly believed in her; her mom for one, and it made her believe in herself. So what if your son pursues this and later on changes his mind along the way for whatever reason...don't let that reason be his own mother. He will never get over it if it is you that dampens his joy and dreams.
I say let him strive! It never hurt anyone. He will either turn in another direction all on his own or show everyone. It's great for a teen to be focused. You don't have to try to determine the final outcome. He doesn't need protected from failure or encouraged to mediocrity. There are enough of us out there. He needs told to just put his whole heart into whatever he does, and that if we do that we are already a success.
No one can foresee the future. Not even those jealous musicians. Wow. That really irks me that anyone said that to you about your child. Music and art is the one area of life that emotional ups and downs actually polishes the craft.
I can just see this star rising. I hope you can catch the vision.
side note ~
I just want to tell you how it was for me when I was six years old. It was during a grade school Christmas sing along that a lady leaned over, got my attention and said to me "whatever you do, don't ever sing". I was shocked. I was probably belting the song out. But after that I quietly spoke the words of songs but I didn't sing. Then later after I was an adult, a friend who was considered to not have a voice for singing told how a teacher taught her the discipline of singing, then she sang and carried every note. Maybe she would never be chosen as a soloist, but she could sing her part, and her voice belonged in the choir. I kept thinking of this, and many years later I began to practice allowing my voice to reverberate, or whatever it is called. I don't know. I'm not a learned singer. But you know what, some sounds I think are beautiful came from my throat, and now I can make my own joyful noise. So please excuse me for jumping on your son's bandwagon and trying to knock some negative others off.