On the simplest level, a stroke is more similar to tossing a ball underhanded than you would initially think. Obviously, your follow through does not go up in the air, but you'd need to be thinking about throwing a ball with very little arc.
Have him stand either at the foot or the head of the table. I suggest the foot, and line up some balls horizontally a few inches off the foot rail and perpendicular to it. (I'd suggest around 8" off the rail, depending up height, arm length, etc.) Then have him stand upright with the cue resting on the rail. No bridge. Just cradle the cue butt in his hand.
Then, pick a corner pocket at the other end of the table. Simply bring the cue back and then follow through the object ball, attempting to roll the ball down the table, directly into the pocket. Don't bend over, don't use a bridge. Try for somewhere between pocket speed and medium speed. (Cue ball is not used here.)
If you can convince him to actually try this (oh, man, this is so demeaning!), you can easily see if he is coming back straight, following through straight (will miss if not), and can execute with relatively consistent, stable speed. It's not just me that thought this useful.
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My memory is foggy, as this took place a long time ago. But, the League Operator/BCA Instructor (Richard) had Jerry Briesath come to town to give a small group lesson one Saturday afternoon. I was lucky enough to have enough cash for the lesson and trusted Richard that it would help my game. Believe it or not, this was the first exercise Jerry had everyone do while he walked around and observed. (Oh, how I thought this was SO beneath me, but I kept an open mind.)
We did a few more exercises while he walked around, then he had us each put all the balls on our table and begin to pocket them as he observed. I was on a back table, where he ended up. I overheard him speaking to Richard, saying "This has to be your best player, right?"
Well, man I got an adrenalin rush from that and was on top of the world...for about 10 seconds. I promptly dogged a simple shot right after I heard that.
It worked out well, and yes, I got enough tips and instruction to maybe add a ball to my game.