Your boy is in the growth and development stages of his life, and as such, muscles and other components of his body are still forming. Now we’d all like to think that your youngster should be like all the others that play this sport - but he’s as individual as he is unique.
Perhaps his muscles look fine under the scopes - but, a few more years of forming and maturing may be just the ticket to avoid what he’s gong through now.
I remember as a youngster I wanted to play hockey because everyone in the neighborhood played hockey. Now this was before all this fancy equipment came out. Back in the 50’s, it was first get a good orthodontist, a hockey stick and then a pair of skates. Well, to make a long story short, I couldn’t stand up on the #@%$! skates to save my life. So, I was a goalie - for about fifteen minutes. I was removed from the game, looking like a peg board. No matter how hard I tried, I just couldn’t stand up on skates. Everyone else could, but I couldn’t.
So look, perhaps, just perhaps, pitching is just not your son’s thing. Perhaps, he could be just as effective in the game at third. Gotta have a good arm at third… and sometimes, perhaps the name of a good orthodontist.