This are great questions!!!
What should he know?
I know I will leave somethings out, so fellow coaches please add on.
Baserunning - lead offs, secondary lead, when to advance on a ground ball at 2nd base, how to read a RHP, LHP, make proper turns, picking up 3rd base coach when going from 1st to 3rd.
Hitting - how to sac bunt, knowing when to show a squeeze, knowledge of the strike zone, good approach when there is less than 2 strikes, to change his approach when there is 2 strikes, to not trust the umpire when he has 2 strikes, to understand that I can have the best hitting coach/mechanics in the metroplex but if I don't get a good pitch to hit I am getting myself out and giving away at bats. Then you'll be giving away either a spot on the team or a spot in the line up.
Fielding - how to play his position(s).
Pitching - know that velocity is not everything...location is, that the knuckleball is a gimmick pitch for old guys that can't throw hard anymore and they need to learn the change up, to understand that when they throw a change up you WANT the hitter to swing, that pitchers need to take care of their bodies especially legs and arms.
Finally, to always hustle!!! You can't always control the final outcome of an at bat or a game. But you can control whether you hustle everytime you step between the lines.
What they need to be able to do?
Pitchers - Have command of their fastball, a breaking pitch and a change up, To work ahead in the count, to throw the ball agressively thru the catcher's mitt (that is a Coach Traubism), trust their fielders behind them and not get on them when/if the make an error.
Hitters - To give the ball force, to give the ball the direction (don't fight the angle, if the ball is away hit it away, if it is down the middle hit it up the middle, if it is inside get your hands thru and pull it), to give the ball line drive elevation.
Infielders - To make the routine play
Outfielders - To take good angles and hit the cut off man.
Catchers - Have good footwork, quick release, don't frame the ball PRESENT the ball to the umpire, and be accurate (throw to the 1B side of 2B).
All this is off the top of my head. I'm sure I will chime in with more!