You have an absolute right to talk to the coach. Your son is 14 or 15 years old. People that tell you to let your child sort things out with a grown man are simply baffling to me. We don't let high school kids have responsibility for saying no to sexual advances from their teachers. Why should athletes have to, with out exceptions, deal with coaches or teachers or other adults.
Adolescents simply do not have the experience, skill, tools, power or authority necessary to handle any number of situations with adults. You as a grown man do. For instance, I'll pretty much bet that the coach(es) don't talk to you as a grown man as dismissively and demeaningly as they did your son.
My son is a Jr. A teacher emailed to tell me that though he's smart and gets bored. Some times he takes out his phone in class when he's not supposed to. She asked if I could help with this problem. I wrote her back that she'd better bet I'll take care of this and any other problem with my son not being respectful of the authorities. If a coach emailed me about my son being a problem, I would want to know what's going on, and if he was a problem, we'd have a little heart-to-heart like CaCogirl did with her son. That is an example of the appropriate time for a kid to learn to deal with "stuff" in life.
But if I think my son is being abused or over matched improperly by an adult. Yeah, I'm getting involved. Baseball, robotics club, home ec. class. Doesn't matter. I am my child's protector when he needs to be protected. You do the same, Mc.
If they called your kid names in an email, that is a problem. I'd want to talk about him being called a "turd" if that's what he was called.
And, BTW. I can sympathize with an incestuous little mind-set that can set in on some coaching staffs, where the whole crew can become little jerks. Usually that doesn't happen. But it most certainly can.
Ps., If your kid is having presentation problems, body language, tone of voice, etc. Get on him now. I've had to do that with my son, because these things matter. I get the sense your son would listen too.
Best of luck.