I've found that I can sometimes get though by taking some time and doing some "ground school", not in airplane theory, but in "what your fingers should be doing" and "what you are seeing" theory. Also, finding a polite way to get the guy talking about what he is seeing and what he was thinking. Maybe get him to talk out loud while flying, describing what he sees the plane doing. Something to get the student to engage the brain.

btw, I've worked with students who "white knuckle" the sticks. A guy here the other day was pusing his thumb down so hard he had a divit in his thumb for a while after landing. You could count the "teeth" from the top of the stick. I made a point of telling him to let go of the stick totally, and watch the plane fly level from one side to the other. I told him "ok, now touch the stick and turn" and "ok, you're level out of the turn, hand off the stick". It helped a little. This particular guy just needs a bit more time to be more comfortable and relax a bit.

ps, I'm not convinced that some folks just aren't cut out for this stuff though. Usually they figure that out on their own though.