Hello, XXXXXX
Does he go outside?
Is there any blood in the stool?
Any trouble passing stool?
Is there any tissue protruding from the rectum?
Thank you for the extra information.
Sounds like he has rectal prolapse. This when the inside of the rectum gets pushed outside the body. Sometimes however it is not the inside of the rectum, but rather the small intestine pushing out. If it the small intestine that is a serious emergency. This require surgery to repair. If it just the rectum, that just requires the tissue to be pushed back in and a loose suture applied until it heals.
You will need a vet to determine which it is. The vet will insert a thermometer into the rectum. It is does not pass freely, it is the inside the rectum. If it does pass freely, then it the small intestine.
either condition does require vet care. The longer the tissue is exposed the chance goes up for the tissue to become necrotic (die).
Keep the tissue damp and moist with warm compresses, or soaking his bottom in warm water. Do not let him chew on the area. If you have to get a cone collar (E collar) from a pet supply store to prevent this.
Usually this is from constipation. Parasites can cause it too, but since he does not go outside the chances of that are slim unless he is a new pet. Adding fiber to his diet will help constipation.
Sincerely, XXXXX
Expert in cat behavior and health
Expert in cat medical and behavior issues.