.... I have helped cure the affliction on several individuals through intense one on one range sessions using the helper load the gun method.
Folks who are flinching may deny they have such a problem, the severity of the denial is generally directly proportional to the severity of the affliction. The shooter load the gun method will undeniably establish the problem. After he has jerked, ducked,blinked,and milked the grip while dropping the hammer on an empty chamber a few times you can have him stand up and forcefully proclaim, I am Joe Blow and I am a flincher. Now the effort to heal can begin.
Tell him straight up, you don't get to load the rifle anymore. I load the rifle, you pull the trigger. The first five or six cycles should be dry, when he is relaxed with good breath control, slip a live one in. Mix it all up where there is no way he can correctly predict what is going to happen. Talk to him, relax, breath, watch the reticle until it disappears. It also helps to stay on the rifle throughout and after the recoil, don't try to see where the round went, close your eyes, lay on the rifle until you are completely relaxed. Slowly open your eyes and take a look through the scope, if you see the target, great, if you don't slowly readjust.
Well, that's my take on the matter, I know this method will work because that's how my dad helped me get over my flinching problem fifty or so years ago.