So do you want to be proactive or reactive? Which one will get the best results? Honestly you and your husband shouldn't have this conversation with the coach because how does this help your son grow up? This needs to be a conversation your son has with the coach. He needs to explain to him how his legs feel after the all day catching session. Your son needs to ask him how he plans on using him if he's pitching first and catching second or vice versa or even pitching at all. Once the coach explains his plan (if he has one) then your son can say he doesn't think he would be at his best if he caught first then pitched (or even did both on the same day). Now the coach can plan accordingly based on the conversation with your son.
Now you guys as parents can help your son prepare for the conversation by helping him understand how to phrase things (working together for the team benefit vs I'm getting my way) and other things like that. But for your husband to go to the dugout on Saturday and create a scene is the worst way possible to handle this. Be proactive, help your son learn to take up for himself and teach him how to communicate effectively. Plus, it helps him look more like a leader in this coaches eyes. Way more good will come from being proactive like this than waiting to the day of the game and creating a scene. Now if your son does all this and the coach still won't listen then I think it's appropriate for you as parents to ask for a meeting BEFORE this situation happens. Do everything in your power to take care of this BEFORE it comes to making a scene. It's not always possible but at least strive for it.