Here are the words that changed it all:
“… College athletes were asked what their parents said that made them feel great, that amplified their joy during and after a ballgame. Their overwhelming response: ‘I love to watch you play.’”
Tell your high school student athlete next time after the game. "I LOVE TO WATCH YOU PLAY." If they were not able to play that day, "I LOVE TO WATCH YOU BE A GOOD TEAMMATE."
That is it, that is all you need to say.