If your horse is really uncomfortable moving, he would act that way either on the lunge or under saddle. See how he acts under saddle on the lunge line and see if he stops to poop a lot. Then get on him and see if he stops to poop. If it is the same with both, then he really may have an issue, either with the saddle or internal. In any case, I would have the vet take a look and make sure there isn’t anything physically wrong. If nothing is wrong, then it’s behavioral and you will just have to make him move and not get away with this particular evasion.
My horse used to stop all the time, finally I had the vet look at him, though the saddle had been fitted to him by an experienced saddle fitter, it was pinching him and making him sore, so he didn’t want to move forward. It was only when I was on his back. Not on the lunge. Of course when I was on him, I asked for more collection, so when his shoulders came up, the saddle pinched. I am only 115 pounds, so it wasn’t a weight issue.