Well, this was a blast from the past! I did appreciate all the good advice from everyone. The conclusion:
He didn't say anything to the coach and tried to pitch through it. He didn't injure the arm any further, but he didn't make the team either, and he didn't get pushed down to JV.
He took that spring off, and his arm felt better. He went to a few showcases and camps over the summer, but his fastball topped out at 85 and he didn't get any attention. Long story short, he worked hard in the fall, grew some more (6'3"/160) played travel ball and made varsity as a senior ...
... in the spring of 2020. And you can see where this is going. He pitched three GREAT relief innings before the pandemic ended the season. He got a varsity letter out of it at least. He chose not to pursue the game any further.
Baseball gave him, and us, a lot. It helped him immeasurably with his self confidence and school life, and it gave us a lot of great memories. I think we're done with the sport though, other than as spectators. His "little" brother (6'5"/230) played, but his season got cut short in 2020 too, and then in 2021 California basically played all its sports in the spring and he had to choose. He chose football and devoted himself to it. Which meant there was no more time for baseball. Summer football showcases, camps, practices and workouts precluded summer ball, and after his senior football season he ended up with a PWO offer at tight end at Cal - Berkeley. I tried to get him motivated for "one last ride" on the diamond with his buddies but he's focused on healing up the aches and pains from football and basketball and then doing speed and strength work related to football so he can hit the ground running at Cal. He's got a good relationship with the Varsity baseball coach though, and he promised me he'd go out and take BP one day so that I could watch him launch a few over the fence and dream about what might have been.
I've enjoyed these boards and this site, and I'll miss the community. Good luck to all, and to all your baseball players!