Hmmmmm, this sure is a tough one. Wondering if you have anything that could act as 3 walls and a roof outside in your yard? Maybe it's the "being indoors" part while pooping that he's concerned about?
Just thinking, if you had 3 large (and tall) pieces of plywood/drywall/whatever would work, nailed them together and nailed another along the top, left it in your yard and tried that? If he would poop in there, take off one "wall" (or side) every other day or so, until there's nothing left and he's outside?
I don't know, may sound stupid but all I could think of.
Best of luck!