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.
How much of an opportunity the kid has to show himself may depend on the off season. He needs to be aware of every opportunity to show himself. In the fall of soph year my son passed on travel and played for the high school fall ball team. This was even though he was on varsity soccer. He had shortstop won by the end of fall ball. Tryouts in the spring were a formality. He had been called the heir apparent since 8th grade. But he wasn't taking any chances. He also attended every winter indoor workout.