I don’t disagree about JV playing time. But the decision is really the V coaches to make. If he wants your son up, that’s probably what’s best for him as the coaches know exactly what the pros and cons are.
I’m not sure who doesn’t choose to stay up?
The HC worries about the varsity team not your son's individual development. "Best for him" is pretty far down on the priority list. If the HC wants a great "scout team", he keeps him up. In the old days freshmen never made varsity unless they were playing. Otherwise you played on JV and got enough high school game experience to help you develop.