It doesn't make you look petty if 9 boys on a team (or their parents) decide the week of the tournament not to show up. That leaves 12-15 boys for a tournament playing 3 games in 90-100 degree heat.
90-plus degree heat is typical in a summer tournament. Giving your son appropriate hydration before, during and after games is simply common sense. If it was too hot for the kids to play, the officials would have stopped the games. Consider it a gift that your son got more playing time than he would have had. There were plenty of other players on other teams who spent just as much time on the field at the same tourney.