I've been following your story and am so sorry that your cat is having such a rough go with his health. Hopefully all the health issues can get sorted out soon now that they are testing and treating properly.
There are a couple of things I might try for the litter box problem:
1) Use dirt/soil from outside instead of store bought litter. If he will use that, then slowly start mixing in a little bit of the litter you would like for him to use. Over several weeks, mix in a higher amount of litter and less soil/dirt until you don't have to use dirt any more.
2) Since he is peeing on towels, try cutting a towel into strips and lay the pieces on top of litter in the box. If he will use that, then same system as above. Start removing pieces of towel every few days until there are no strips of towel and only litter left.
It is entirely possible that his litter box problems are largely due to the urinary and other health problems. He may start using a box once these conditions are all cured or managed. (Plus neutering should help for both the cat box and yowling-and-prowling issues.)
There are a couple of things I might try for the litter box problem:
1) Use dirt/soil from outside instead of store bought litter. If he will use that, then slowly start mixing in a little bit of the litter you would like for him to use. Over several weeks, mix in a higher amount of litter and less soil/dirt until you don't have to use dirt any more.
2) Since he is peeing on towels, try cutting a towel into strips and lay the pieces on top of litter in the box. If he will use that, then same system as above. Start removing pieces of towel every few days until there are no strips of towel and only litter left.
It is entirely possible that his litter box problems are largely due to the urinary and other health problems. He may start using a box once these conditions are all cured or managed. (Plus neutering should help for both the cat box and yowling-and-prowling issues.)