If the back hurts when pooping or because of the pooping posture dogs might hold on and not poop. In my experience delayed poop ( my fault, late walk) results in loose stool but I've heard others says it results in hard poop for their dog.
I would be very leery of giving anything that might soften stool to make it loose before first being sure there is no blockage. Loose stool behind a blockage is going to give the dog agony. Pumpkin is given when poops are too loose or too hard but they are appearing, not when there is no poop.