Very sorry for Zack since I share his love of baseball.
It is unfortunate that coaches can be so callus, but it has always been so.
Best thing, in my mind? Use this as a great opportunity to explain that adults/authority figures can and do make mistakes, that he has done his best and that is where the value is in all sports, doing your best unencumbered by the opinion of others.
Honestly, we all know when we have given it our all. Your post makes it clear Zach is one of those kids. Focus on that.
I didn't express myself well in this post...just let him know that his effort has value beyond the opinion of his coaches
And muss up his hair for me as well.