Assuming all this is straightforward and true, encourage your son to focus on those things he CAN control.
As in, work hard and be ready when your chance comes. He may well be # 7 on the depth chart, but if the others are as weak as you say, at some point he'll be in a game or two. And if he gets them out, he'll move up the depth chart.
If he doesn't, he won't. So, the challenge is on him. Be ready at all times because when you're in the bullpen you never know when your chance will come. If you blow it, you may not get another chance.
Being ready means doing everything right every day. Eat right, get sufficient sleep, work out and practice with purpose, comport yourself as a model teammate at all times.
It's a long way from JV to the end of high school. He wouldn't be the first kid to open some eyes and move way up the ladder before his time was done. He also wouldn't be the first to muff the chance through focusing on grousing and blaming instead of his own responsibilities. Right now, he just needs to decide which of those forks in the road he wants to take.