Cats will defecate outside the box for many reasons. Here is a good article to explain the different reasons:
https://www.petcoach.co/article/the-top-5-reasons-your-cat-is-avoiding-her-litter-box-and-what-to/That is good that Tyler has already been examined by the veterinarian. If the veterinarian thinks this is behavioral, there are a few factors to adjust.
Cats are sensitive to the type of litter, size of the box and location of the box. If you have more cats, you need 1 litter box per cat + 1 additional box. These litter boxes should be as far away from each other as possible (in different rooms is the most ideal). You should also have a litter box on every floor of the house. This article discusses different types of litter and litter boxes:
https://www.petcoach.co/article/cat-litter-finding-the-right-fit-for-your-cat/In general though, cats prefer large, uncovered litter boxes, clay-based clumping litter that is scoped every day and a private, quiet location. If you have other cats or dogs, if they can access the litter box, it may cause Tyler to be afraid that he could be trapped in the box. Cats are a prey species in the wild and they still retain those instincts.
Clean the soiled areas with a good enzymatic cleanser. This will to get rid of any smells. There are things you can do at home to help to keep Tyler relaxed and that may help to ensure good litter box habits. Here is a great article about cat's instinctual needs:
https://www.petcoach.co/article/why-every-cat-needs-a-place-to-climb/Use a Feliway diffuser, set aside 30 minutes a day for vigorous play and/or switch the food to Royal Canin Calm. Most cats response best to combination therapy. The veterinarian can also prescribe medications which can help with house soiling. I hope this helps and please feel free to post any additional questions.