Welcome to the site. You are getting great advice from many who have been there. I certainly agree that your son should be taking the lead on talking to the coach, asking him what he needs to do and what areas of his game need improvement to earn playing time. And, then, more importantly, he should work his butt off to earn what he wants, striving to eliminate any doubt in the coach's minds.
That said, I have a slightly different take on your involvement. Many here say "stay out of it". I think it is important that you ARE very involved. Be involved by setting the good example at every opportunity. Be fully supportive of the coach whether he is playing your son or not. Be a big fan of every kid on the team and cheer equally for your son and the player/s he is competing with for playing time. Be the parent that doesn't fall into the trap of criticizing other players to put your own in a stronger light. Be the parent that has a supportive response to the negative banter of others. Be the parent who doesn't try to make excuses for your son. Be sure he knows that there will be new challenges to overcome every step along the way. Offer your son guidance on how to take positive action to overcome obstacles. Understand that all parents have slightly rose colored glasses upon their own of one tint or another. Know that other parents will be inclined to tell you "your son is really good and he should be playing instead of so-and-so" as a courteous jesture just as they would say "hi how are you?". Understand that the coaches are with the players 3 hours a day every day and they really do know more than anyone what each player can contribute to the team (I realize there are exceptions - in that case, see previous "overcome obstacles" comment). Be the parent that always has something nice to say about any of the other players and it doesn't have to have anything to do with baseball or playing ability. Understand that this environment is a hotbed for your son to learn life's lessons. Don't under-estimate the fact that so much of what he learns will be based on his observations of your behavior in these circumstances. So, IMO, don't stay out of it. As he strives to be the best he can be, make sure you are on your best parenting game.
At the end of his four years, what will really be the important take-aways for him?
If you are able to keep this perspective, I think you will both find the game at the HS level to be very rewarding and enjoyable.
Best wishes