This was 4 years ago, but I don't think it has changed much. Pitchers tryout in the bull pens with coaches watching. There are around 8 mounds in a row that the coaches are watching. There will be a primary coach with a gun at each mound grading the pitchers. You basically have to be in the top 50% to be selected for the game based on the numbers. They are not pre-selected as kids are coming from all over the country and so are the coaches so I am not sure how they could do this.
You should hope for a lot as it is one of the top high academic events in the country as nearly every coach at the Sr Classic will be there. Even high academic schools are looking for good ball players, GPA's and test scores get you looked at but the are looking for talented players. Have him relax and be himself, being a LHP is an advantage. Good luck!