The umpire had the right to tell either you or your player to take care when dropping the bat. There was no need to discuss the "what ifs". He should have also not implied he was watching the clock. That was unprofessional.
NFHS rules sate that once players enter the batters box they must keep at least one foot in the box in between pitches so unless time was called or a foul ball or a swing and miss he has the right to keep the batter there.
Coach, if you want to make it past freshman ball you will have to learn to control yourself. In HS ball the field is an extension of the classroom. A teacher would not be allowed to behave that way in a classroom and you can't do it on the field. There might be some extreme instances where you get into it with an umpire and it would be considered part of the game. In this case you "lost it" with little provocation and it is unacceptable. Take this as constructive criticism and learn from your experience