I agree w free, I would make sure he either was wearing a break-away halter, or was tied w something that can easily break.
B/c he sounds very anxious about the pressure, the following might not be for the poor fellow, but if you can monitor him you might try putting a halter on him w a very short lead on it such that it does not touch the ground while his head is in its ordinary standing position (a few feet long, depending on how tall he is) - and let him amble around in a corral. Usually, after a bit this helps them learn the idea of going not panicking and going into the pressure when they inevitably put their head down and step on it - and it "catches" them.
I looked at this thread b/c I am starting to teach a 2.5 yo to stand tied that has no prior "stand tied" training. Unfortunately, at this late date she is rather powerful and I actually worry about what damage she could do to herself and stuff. So, I am going slow. I stand behind a rock wall w the rope so I have the leverage of the wall, and my hand stays stationary and acts as the "tie ring". I wait for her to decide to go "out of range" (this can take a boring long while), she eventually pulls, and just "leans" (think donkey) pulling, she thinks it over and comes forward. It is an entirely "controlled" excercise and she can't hurt herself. She is basically ready for phase two...to be tied for real.
I have visions of things collapsing if she pulls on the post w the power the little tank can muster. You might try to go back to square one w him.