An old tried and true method of helping correct flinching and yanking the trigger is called the "Ball and Dummy" drill.
On a revolver, load 3 live rounds and 3 snap caps (or fired cases) in a staggered order and let him shoot.
On a semi auto, stagger live ammo with snap caps.
He'll quickly see that he's jerking in anticipation, or yanking the trigger, and it'll help him slow the trigger squeeze and concentrate on his sight picture.
On a semi auto there's the additional benefit of learning the malfunction drill (tap, rack, reasses, bang).
The mantra of every Firearms Instructor is smooth trigger squeeze, and focus on the front sight.

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