Most coaches aren't stupid. Typically they can judge talent. They see a lot more than the parent sees. And they don't see it with bias. If it appears the coach has a favorite player it's typically one that works the hardest or is the most talented.
While somewhat true, I would caution about how much a HS coach actually sees. If you just do the math, coaches have very little time before games begin to evaluate players. Therefore, you could be looking only at 20-30 hrs. of practice time. With maybe 20 kids on a team, that is at best about an hour of actual skills time. And in many cases, the penciled in starters will get more time than the backups.
So not to put pressure on the kid, but realize that there is a very small window of opportunity to impress a coach and bump a "starter." He may only have a handful of pitches to show he can hit, or a few balls to show he can field. So he has to be ready to perform. That is where the arriving early, staying late, and busting a$$ can play a part.
I would add that Winter Workouts and Summer/Fall Ball, if the team has it, can have an effect too.