[MizunoJoe]

Is it simple if it doesn't work? I suspect he has tried not to do it, he knows he shouldn't. If the movement is habitualized and he isn't AWARE of the proper movement he will never change. My suggestion eliminates the shrug and replaces it with the proper movement. Albeit forced open, the door is no longer jammed and he can then learn to swing it freely.

Bumpy

And I suspect he hasn't tried to take the tension out of his shoulders, which is a simple thing to do, and you don't know that it wouldn't help. Taking tension out of the shoulders is not the same as telling him not to shrug.

Shoulder tension is one of the most overlooked swing wreckers, and even if eliminating it doesn't eliminate the shrugging, it's still a plus.