My name is***** and I am so sorry to hear that P. Kitty doesn't like his litter box.
What diets have you tried with your kitty?
Have you tried psychotropic medications such as amitriptyline?
How about Adequan? This is a joint supplement typically used for dogs, but it can also increase mucus in the bladder, improving bladder comfort in cats.
Have you tried herbal calming remedies such as Composure (available over the counter from Amazon)?
Have you tried offering different substrates such as area rugs, different types of litter (clay, corn cob, clumping, etc), potty pads?
How about arthritis? Does he maybe have trouble getting in and out of the box? Have you tried pain medication? Steroids?
Lastly, have you considered a behavioral consultation with a veterinary behaviorist? They may have some other ideas to improve box usage.
Are any of these ideas new? Do you have any other questions?