You can't take to heart what other children are saying because if they're not getting as much time as they think they should, of course they're going to think it's a conspiracy. Talk to the coach, ask him what his prerequisites are for putting a player in, what the requirements are for putting a player in and what his thoughts are on your sons game and abilities. Sure some teachers can be jerks and some of them (particularly coaches) can be very specific about who they put in because they solely focus on winning games, as opposed to giving fair time to each player. So start out with the coach and see where it goes from there. If the coach won't work with you, you could always take it to the next level by going to the principal or the board of ed, but think long term, and unless he's trying to get a scholarship to college based on his playing it may not be worth making a whole lot of enemies in his new school, and having the coach possibly take things out on him.
I welcome your thoughts, let me know if you want to talk more