Welcome to the site.
What should you do? If your son loves to play the game, encourage him to always work hard to be the best player he can be. Encourage him to always be a good teammate, regardless of the efforts and attitude of the others. Never allow him (or yourself) to fall into the thinking that it is coach favoritism, whether there is any degree of truth to it or not. That will just take away from where his focus needs to be and enable him to be an excuse maker going forward. If he hears you talking about coach favoritism, he will follow your lead. Don't go there. Set the right example.
There will always be hurdles for every player to overcome. Politics, injury, seniority, depth at your position, lack of skill set, conditioning, age difference, lack of access to good instruction, budget.. the list is endless. The message should always be to work hard enough to jump the hurdle... and then start working hard again for the next. Because they will keep presenting themselves. That's part of the beauty of it. One of the objectives should be to work so hard, your skill set improves to the point that you leave no doubt for any coach that you should be on the field, no matter the circumstances.
You didn't say what age the players are. I suspect on the younger side. You have come to the right place. You can also search threads by topic. The other thing you should do is USE THIS INVALUABLE RESOURCE YOU HAVE FOUND!