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Is he eating?
Is he straining to poop?
Constipation is very uncommon in dogs, but it does happen. Because he has stopped eating and is actively straining to poop I think you should take him in. They can do an x-ray to see how backed up he is and administer a proper enema. An enema for a 115 pound dog would be a lot of fluid, and not something you would want to do at home.
The vet can also feel inside the rectum to make sure there is not a reason for the blockage. I have seen bones in the rectum that lodge and prevent pooping or polyps or growths etc.
If he was eating I would tell you to increase fiber, but if he is not eating he has to be seen by the vet.
I can also say that sometimes straining to defecate also means they are about to break with actual diarrhea, but your vet will be able to help with that.
If he were eating I would say give to tablespoons of pumpkin with each meal. and that would bulk up the stool and make things easier to pass. The tramadol usually causes less frequent bowel movements but not usually constipation .
You can also use miralax. a capfull in the food once daily along with the pumpkin, but again if not eating i suggest he go in . If he will eat boiled chicken and white rice and the pumpkin and miralax that may work .