Does coach owe explanation?

I'd say the coach owes the player an explanation when the player asks specifically.

Although your son is doing the right thing by handling it himself without parental involvement, there is also a right and wrong time to have that discussion. Coaches are generally not very receptive to conversation just after a game. I would suggest your son calling or better yet, emailing the coach and making an appointment to talk.

Good luck to your son in his discussion and in season.

Here is how as a coach I would have handled this situation. When you make a decision to go with another guy you owe that player an explanation. Why? Because its the right thing to do. You tell him why you are going with another guy. And you tell him what he needs to do in order to be the starter again. And you tell him to continue to work hard and keep a good attitude. And then you tell him its up to him to win it back.

Now this coach has decided to go about it a different way. Thats fine its his team and his decision to make. The player should go to the coach before practice and say "Coach if you dont mind I would like to talk to you after practice." Ask the coach what you need to do to get playing time. Tell him your willing to do whatever it takes to help the team win. And then go out and win the darn job back. Good Luck.

has your son done anything off field the coach may have heard about? this still isn't the way to handle it ,but it would /could be a reason?

but he should at least tell him why it's happening.