Here's our advice for whatever it is worth. First, let your son work it out. You mention that the coach has cussed out other parents. He is the norm, not the exception. I don't know of any coach that wants a players parents questioning his playing decisions. Parents need to stay out of those type of issues.
Second, being on a baseball team is more than just playing or starting. Everyone on that roster has a role. I'm sure if your team is like most other high school teams, they have mid week scrimmages, and situational practices in which everyone participates. Those scrimmages and practices are just as important as the games to many coaches. If your son is good, then his skills may motivate someone else that doesn't want to lose their position to your son. You'll have to ask your son, what motivates him? Is he setting goals and working harder in practice to achieve those goals? I only ask that because you also mention that:
"He's not learning anything, and he's not developing his skills at all."
Again, that may be the problem at hand. Because practice and scrimmages are where you learn and develop skills...it's not always in regular season games. The coach may have picked up on that. I don't know your son and for all we know he is the hardest working player on the team, but the fact that you mention he isn't learning anything makes us wonder if maybe that isn't being projected to the coach. If that is the case, then your son has already "quit" in his mind.
The only way to turn this situation around is giving 1000% effort, learning, developing and setting goals to get out of that mental rut. If he truly loves baseball, he needs to truly love the journey to get there as well. That means paying dues, working harder, hustling and doing everything he is asked to do and more in practice.
There used to be a kid we knew who was a smallish infielder who also never played much...but he never gave up trying. He was the inspiration in the dug-out, leading the team in other ways like spirit and upbeat chatter on the bench. He usually only got in games that were blow-outs, but when he got up to bat, he received more vocal support from his team mates than anyone else. He never even thought about quitting. He was having fun just being on the team, with his friends and for the love of the game. At the end of the season, the coach gave him a special award for being the most inspirational player on the team. He never played much, but I guarantee that he learned and developed in many ways other than just baseball.
Quitting is usually not the answer. It's kind of a disease in our opinion. Once you do it once, it's easier to justify doing it again and again.
Have your son turn it up a notch and see what happens. The coach hasn't cut him, he is on the roster and he must see something in your son right? If things still don't change then at least your son can hold his head high and at least know in his mind that he gave it his best effort.