Funny! But not, I know. I suppose he could be waiting so that he receives praise for using the box, and this time he said "uh, no, gotta go".Here is what I wonder: he has NOT pooped in the box in between visits. I am wondering if he's "saving" it for when someone is there so that he gets a treat and those occasions when he poops outside the box, is it because he can't hold it any longer and then just poops where he happens to be? And if that's the case, how is this going to work assuming we're able to let him out?
Have one of you been in the room at the same time he has actually gone in the litter box over the past 4 days? Tbh, that would be the only way he could really even come close to correlating a treat with using the box. Literally, a single minute or two could pass from the time he uses the box and gets a treat, and he would have no idea he was being given a treat for pooping in the box.
Once he gets the pattern down of using the litter box, I am not sure the treats are really going to matter. Other than just because you LOVE him!!
Also, in that space three boxes are a bit much. It sounds like he uses all three? You might want to reduce it to two, and see if that makes a difference.