Adult male poops outside of litterbox frequently

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Ok, on to Bear: A couple quick questions.
What did the vet check for to determine that nothing was wrong? Chronic soft stools, and especially loose ones, are not normal. The cause for this needs to be determined and treated. I don't know if you've ever had a really bad bout of diarrhea yourself, but the cramping, discomfort, and soreness that goes with it isn't something that a cat needs to chronically go through - you'll see litter box avoidance if you're not lucky (as is the case).
Soft stools can be anything from an underlying protozoa/parasite or bacterial infection, to food allergies, to chronic inflammation. These have the possibility of making a cat very ill and even causing death, so please don't take him to a shelter where they either won't or can't afford to treat him properly.

Did the vet check for urinary problems? It sounds odd, but that can cause cats to poop outside the box, too.

In the meantime, while you're dealing with possible health issues, add yet another box. Scooping another box is better than scrubbing the floor.

I would have the vet check for giardia or coccidia. Those are hard to find and sometimes will only be seen 3 out of every 10 times they look! Has he been on Metronadizole or any other meds for loose stools to see if this helps?

Please don't scold him - all that will happen is he'll think pooping itself is bad and will try to not poop at all, or poop in hidden places - neither of whichi s good. Praising can work, but often there's too much time between using the box and being praised - he's not quite sure what good thing has happened.

I'd get a second opinion from the vet...my boy had poo issues outside the box. In his case, a course of Tylan powder helped immensely. I believe in his case food was a culprit - Dante doesn't do well on too much wet food (go figure) and the fishy Hills CD wet just wasn't working for him and was upsetting his tummy. Also, has your boy had blood tests, urine samples, all that good stuff - a lot of times, as was said, a UTI can also trigger poo issues.

I also added a box to the area where he was having his accidents. I'd definitely have a couple of boxes, even for one cat, and I believe it's worth trying Dr. Elsey's Cat Attract in at least one box.

Is he pooping right outside the box? Or far away? If it's right outside the box, I definitely think getting a brand new box, and a different litter, like Cat Attract, is worthwhile....he may simply no longer like that litter, or the manufacturer may have changed something in the litter over the years.

In my experience (and believe me, I feel for you), cats go outside the box as a way of communicating something is simply not right in their world - they're ill, or they hate the litter, or the box is too icky for them (whether we think it's icky or not - scooping twice a day is a must in my house!).

Oops, hey - have you cleaned the area with an enzyme cleaner - that's a 'got to' as well. Otherwise, the scent can draw a cat back to the same area.

I have a kitten who ocasionally poops next to her box and has chonic very soft stool for months. The vet did test urine including cultures plus tow different fecal analysis with corss referencing tests. She has been on antibiotics, probiotics and now Tylan powder.

The thing that stumps my vet is that she is gaining weight, playful and clean. She does have a slightly firm belly.

She does small better than she used to.

Have you started ruling out food intolerance or allergies? Generally if nothing else is found to be the cause of it that's the next step. Some cats can't handle certain ingredients - I have one that can't keep down corn and wheat, and another that gets terrible diarrhea from chicken.