My name is***** and I am sorry to hear that your male kitty is pooping outside of the box. There are several reasons why this could be happening:
-arthritis- sometimes cats have difficult time positioning in the box to defecate and they will position so their rear end is hanging out. To help with this, I recommend a very large box, like an under the bed sweater box with no lid. This allows for more ease when positioning for defecation and can help cats with rear end arthritis.
-inflammatory bowel disease- this is trickier to diagnose, but I have seen inappropriate defecation as the first symptom.
-bladder infection- this is also less common for inappropriate defecation, but possible.
-other metabolic problem such as hyperthyroidism.
I would try offering a third much larger litter box as the first step. If that doesn't work, you may need to have him examined by your regular vet and consider blood work, urinalysis and x-rays.
Best regards, ***** ***** this answer helps you.
Feel free to contact me if you have any more questions.
Kansas State University,
College of Veterinary Medicine Class of 2001