You have been given good advice, it's time for you to take a step back and let your son figure it out for himself. He doesn't have to speak up if he doesn't want to, I am sure that the coaches know that all kids want to make varsity, only they will determine when and if he will. Not speaking up has nothing to do with lacking courage when on the mound. When my son went off to college, nothing more he wanted to do than be a weekend starter, I am sure they knew it, but you know what, he wasn't ready for it, yet. Your son is probably very good, may be more mature physically than others, but that doesn't mean he will make varsity, there is still a lot of growing up to do.
Why do some feel the need for our kids to rush through the game?
I don't beleive that freshman should begin in Varsity (unless they have to). I beleive that there is a transition period that is important for most players, has nothing to do with their size or their physical abilty or what they did before they entered HS.
It's also often about paying your dues, and in baseball, there is one thing you need to learn is about paying your dues.
My son was more than capable of playing on varsity when he entered into HS, they placed him in JV and he loved every minute of it, he got to do it all, when they moved him up he was a pitcher only and he wished he could go back to JV.