In all honesty - if this is the route you go, I'd have him do it once a day to get into it a little. It's easier to practice some form of mindfulness/meditation/grounding when you aren't at the height of anxiety - which then makes it easier to do when you are anxious.

And there are a lot of options. Have him take a swallow of cold water and tell him to see how long he can feel the cold water going down his throat. Then take another. Some people put a piece of ice in their hand and feel the cold while watching it melt, etc. There can be some stuff that seems pretty crazy - but it's all based on getting grounded in the present and now and out of all of the anxiety producing thoughts of what may happen. There's actually some pretty good free meditation/relaxation apps out there than you could get on your phone and have him listen to on the drive to the game.