Not sure about any particular drill, unless you can get a live batter to hit popups in practice
About all you can do beyond discussing the physics of a pop up is a lot basic popup practice. One drill that you might consider uses a tennis racket and tennis ball:
1. Find a clear area in the outfield
2. Put on all the gear
3. Place the catchers glove on the ground in front of his feet
- The catcher is now holding a tennis ball in one hand and a tennis racket in the other
4. Catchers hits himself a high popup, squats down and then places (doesn't throw it) racket on the ground in front of his feet
5. Catcher then picks up mitt and puts it on, removes mask (holding it in his hand)
6. Catcher begins locating the ball on his own (the "savy" catchers remember the angle the ball came off the racket, just like a baseball)
7. Catcher locates ball and then disposes of mask in a "safe" area
Why use a tennis ball?
Because it's very difficult to "one hand" a tennis ball popup thereby forcing the use of two hands PLUS you can consistently get a tennis ball up to a desired height that allows enough time for steps 4 & 5.
Why not hit the ball for the catcher?
Because this is a drill he can do himself on his own time IF he is motivated to be a good catcher. If he can't or won't do this drill on his own perhaps he is playing the wrong position.