This is part of life.
Tell him that the coach has the confidence to start him he should feel good about that. Your grandson has played a lot of football. Tell him this game is like any other game and to be enjoyed and just try to do the best he can but HAVE FUN! He was chosen to do this. His coach chose him and God chose him via his coach.
He was meant to play in this game. He was meant to start and there is a reason that only God knows and he should trust that. Tell him to show courage and poise no matter how the game plays out. If he does that then everyone will respect him and trust him as the starter.
Just play the game he's played for a long time. ITS STILL JUST FOOTBALL! Have fun and play like he knows how. Trust himself. Believe in himself.
Grits