I agree with what's been said about the captains approaching coach first, etc.
From your end, it's probably a good time to explain that coach probably has a misguided sense of how to "motivate" players, he clearly doesn't understand the kids, and encourage him to shake it off and not take it personally. I'd encourage your son not to quit, no matter how much he might want to, and I don't say that lightly. If he loves the sport, he'll regret letting the coach run him off of it. Happened to me with football, and to this day, I wish I stuck it out another season.
We've all run into similar bosses, interview subjects, clients, customers, etc., as adults. It's imperative that he learn how to deal with them and/or the processes to have them removed if it gets to that point. This is a relatively low-stakes situation to do so.
EDIT: Obviously, it's his decision to quit or not, and let him know it's a valid course of action. Just make sure he understands he can't get back his senior season if he chooses to quit. The coach isn't going to suffer by him not being on the field. Your son might suffer if he realizes he made a mistake by missing his last season.