Well, constipation or diarrhea can cause what you describe. Heck diarrhea or constipation can cause bleeding to humans. I should know, heh. There are also other health conditions like Inflammatory Bowel Disease, Chron's Disease etc which are diet related and can be controlled by switching to proper nutrition.
Now, back to the constipation topic. Is your cat regular? Canned pumpkin is a stool softener and can certainly help with that, but we first need to be sure your cat is or isn't constipated.
He could be bleeding through the rectum but he could also have blood in the colon or nearby areas instead. Having blood in the colon is bad, what can I tell you, and it can be seen when the cat goes to the bathroom. Inflammation in the rectum can be caused by food allergies, again, constipation, diarrhea or bacterial infection. You want to take the cat to the vet to get a diagnosis, and it might be a good idea to take a stool sample to the vet with you. Hopefully, your cat's problem is he is having a bit of a hard time evacuating. If there is some sort of inflammation in the rectum or colon, then antibiotics, maybe prednisone and a change in diet can help your kitty feel better.
Let us know how this goes, and good luck to you both.
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