Unfortunately the matter is typically somewhere in between what Fungo is saying and what beenthere is saying.
First, it sounds like your son may not be getting the playing time he deserves but it doesn't sound like he's getting blackballed either.
As far as what Fungo's comments imply I've seen talented kids work hard, keep their mouth shut, support the team and sit their entire HS "career". I've also seen kids do that and get an opportunity when someone got hurt. There are no guarantees. Some do get the shaft, some get lucky. Even so, Fungo is right that talking to the coach is unlikely to help. Is that right? No. Is that the way it is? Yes. Are there other parents expertly kissing up? Almost always. Guess what, if you've gone and talked to the coach you aren't an expert and never will be so don't even try.
As far as beenthere's advice goes it is still probably better to put up with a poor HS situation than to quit and there are good things that go with HS baseball even for those who don't get their fair share of playing time. Quitting puts a tag on you that will take even more talent and hard work to overcome. The important point here is that you shouldn't tell your kid that if he works hard things will always work out. Chances are better but sometimes life just isn't a fairy tale and the "good guy" doesn't always win.