Biggest thing I will say is the elbow needs to be in front of the knee. In the front but to the side is workable also. At the HS level, pitchers aren't that good yet and if the elbow is outside and behind the knee, you won't consistently get to balls in the dirt to your backhand side. If the elbow is in front of the knee you can clear the knee and still get to those pitches. At the same time you never want the elbow back inside the knees; that is death for a catcher when the pitcher misses his target.
Not saying to catch with straight arm or reach out for pitches, simply starting elbow position. Elbow can move back with the inside pitch from this position also while allowing outside coverage. Got to have the mitt in a position where making the transition to blocking is quick and smooth, something all HS catchers will do alot of during their career. Personally like about a 30-45 degree elbow angle, ballpark.
The coach obviously wants the catchers to let the ball get deep. Problem is many coaches interpret deep as between the knees and chest. Just doesn't work for most catchers and I strongly believe that method has a tendency to create a passive catcher that will have difficulty consistently blocking any ball that isn't straight at him.
Watch MLB and college catchers and where they position the elbow compared to their catching style. Your son needs some clarification as where the coach actually wants him to catch the ball before saying the coach is teaching a skill that doesn't work. If the coach is a between the knees and chest guy, I can almost guarantee the catchers will not be very good at blocking due to the quality of pitching in HS...even the great HS pitchers make mistakes. Passed balls/wild pitches lose as many HS games as anything.
Target...open mitt wide during delivery but then 1/4 turn, slight drop, whatever to relax the hand/wrist. Once the ball is in the air the target isn't important. Beating the ball to the spot is. Gotta be smooth, fluid and quiet behind the plate.
You'll see a variety of opinions of this; some similar, some in total disagreement. All I'll say is you can't cookie-cut catchers as far as style; they need to find what best works for them so they are smooth and natural and never rushed or jerky.