That is tough if some ringers come in at the last minute, and take some of your son's innings. I've known some kids who've been on the opposite side...ringers who flew in to pitch. All I can say there is out the team after you have left, if their actions are outrageous.
In some cases it's justified. A local travel team with a very good rep tends to have really small teams, so as to give everyone lots of innings, but in tournaments where they are going to be playing lots of games in a short space of time they need to have extra arms on hand.
Other teams have rosters with 18-20 players. Strikes me as a cash grab.