It's up to him. Baseball, especially at 12 is supposed to be fun. And if he isn't having fun, he won't want to play at all at some point.
Pitchers are a special breed. My son pitched and played position all the way thru HS. But, pitching was his thing. I would often ask him if he had the choice between pitching only every few days or playing position every day, his response was "why would I not want to pitch?" That's his mindset. He didn't have to make that decision or accept his role one way or the other until college, but he is doing what he loves to do - pitch.
If a kid doesn't want to pitch, then he shouldn't have to, in my opinion. Like I said, baseball is supposed to be fun. If he loses his love for the game because his is forced to do something he doesn't want to do, that is not a good thing.
Others may have a different opinion, but that's what I think.