Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian.
I understand your concern about Tyler's behavior. It's no fun to find stool where it shouldn't be.
Cats stops using their litter box for a few reasons:
1) They don't feel well and associate pain with their litter box so go elsewhere hoping not to experience discomfort. This can be gastrointestinal pain, degenerative joint pain, or anal gland discomfort for example.
2) Their litter box is dirty, hard to get to or in or out of, they don't like the litter in the box, or they don't have enough privacy in it or feel trapped in the location it is in.
3) Other cats (can be indoors or outdoor strays), dogs or people are scaring them when they are in the box or trying to get to the box.
4) Social stress and overcrowding. Cats in the wild do not live together. It is socially stressful for them to have to be confined together. That is likely why many cats stop "marking territory" once they are allowed to go outside. They are less "socially
stressed" because they have more room and get more exercise.
First I recommend limiting access to the areas that he is inappropriately using as a place to eliminate as you are currently doing.
The longer this goes on the more it becomes a habit. If this has been going on for a while it is likely that we need to find and treat an underlying problem that started all this as well as retrain him.
I am glad she's had a physical examination to make sure all is well. Make sure though his anal glands were checked, and that he doesn't have parasites (check a stool sample) or spinal arthritis that makes it painful for him to go or maintain his position. If he ever has blood or mucous in his stool, it is soft, hard or very large or small and difficult to pass he may have inflammatory bowel disease, constipation or
megacolon which are all uncomfortable. We need to address any medical problems
to have hope of retraining him successfully.
You also need to make sure that the area that he has picked to go has been cleaned with an enzymatic cleaner, like Nature's Miracle. Although it may smell fine to you their noses are much better than ours and as long as the odor is there he will be attracted to the spot.
Make sure his box(es) is/are spotlessly clean, scoop stools daily, change litter completely weekly and clean the box itself. If the litter box is older than a year get a new one. Many cats don't like odor and an old litter box stinks to them. It might be fine to pass urine there as that is quick but stool takes longer so they are more
particular. Ideally you need one more box then the number of cats so that he has a choice and there is no litter box guarding. They should be in different locations and where he cannot be bothered and they are easy to get to. Some cats like to urinate and defecate in different areas and are very sensitive to being interrupted. You may have done some of these things already but sometimes you need to repeat them and I list them all to be complete.
Make sure she has privacy when he goes, yet also make sure the box is easy to get to and get in and out of. Many cats appreciate low sided boxes, especially as they age. Some cats also like a bigger box to pass stools in so they have plenty of room. The plastic very low sided storage containers that fit under the bed work very well.
If you are using scented litter I recommend plain clay litter or plain scoop litter. Most cats find scented litter objectionable.
To help ease social stress you can try using Feliway sprays or diffusors. These are synthetic pheromones which mimic those produced to mark areas as safe and many cats find them soothing. You can also use pheromone calming collars as well. See this link for some examples: http://www.amazon.com/Sentry-Behavior-Pheromone-Collar-Inches/dp/B0026JAKWG
If these measures aren't enough you can try a homeopathic calming oral medication called Bach's Rescue Remedy. See this link for further information: http://www.bachflower.com/Pets.htm
And you could discuss oral medications with your veterinarian as well such as fluoxetine or amitriptyline as calming agents to decrease his stress.
If you do all this and he is healthy and he is still not cooperatingI would confine him to a large dog cage or small bathroom with his food, water and litter box to retrain him. You can let him out only when you can supervise his behavior. If you catch him going to his "spot" use an air horn to scare him. He must have the negative
consequences every time or this won't work. Do this for several weeks until he consistently uses his box. Then slowly give him more access to your home, a
little more area each week if he continues to behave.
Best of luck with this fellow and please don't take his behavior the wrong way. Cats that are of this age and suddenly change their behavior almost always due so due to a medical problem and it sounds like he is trusting you to get him some help.
I will be on and offline all day so please feel free to reply with any further questions and I will get back to you today.