So much of what Grits said is the truth............
I was a varsity HC for baseball, not football, for years. I get the pressure these kids are under when they reach this level and such a game. As a HC I got it constantly from parents, administrators, and parents booster club. My two nephews are playing this Saturday in the semi's also here in NorCal as a 2nd seed in their playoffs. They are a premier program from a small town expected to win. A starting RB right now in the NFL came from our small town and played for the same program. Mind you neither is playing QB, they start at DT, DE, and long snapper, but one can tell they are feeling the pressure.
First and foremost it's up to your grandson's coaches to place him in a position where he can be successful this Friday early which will help him relax. I suspect they will do such. Also like others have stated this is happening for a reason. He is suppose to be the starting QB this Friday. So I know it's hard but he needs to be told over and over again to just relax and just go play. And have confidence in himself. No negative thoughts. And whatever happens that the adults in his life remind him that the world didn't stop because of some football game and what a great experience for his next two years. Have fun! Hell not many get to play in their HS semi-Finals playoff game.