When I managed a humane society in my former life, we'd have quite a few nervous poopers... sometimes when owners were surrendering their pet and we'd be walking them down the hallway, away from their parents, towards the kennels; they'd get really nervous and balk and poop would literally just fall from them... At other times, when we'd put the dogs up on the table for a vet exam, they'd lose control of their bowels and just poop (not loose, just poop)...
I also have witnessed many, many, many horses when nervous about going into a trailer, stall, whatever; will have insta-poop right on the threshold of what they're scared of/nervous about...
I don't know what to do to resolve it other than, as was mentioned, desensitization... I know he's not the only little guy with the problem, though!
Good luck!