My Male Neutered Cat Keeps Spraying?

This is my only cat. He is 9 years old. I have had him since he was a kitten. He is neutered; it was done when he was 2 years old. He uses the litter box and marks. He also has access to a fenced in patio it is like a pet run. Only about 22 feet long and 6 feet wide.

There are feral cats marking the outside of the fence near a bush and he smells it and marks outside which is fine by me. But then he also does it inside.

I live in an apartment on the bottom floor; it is a cave as it is. Am I supposed to board up all the windows so he can't see the cats and never let him see the outside for the rest of his life? And I am supposed to live in darkness for the rest of his life?

I clean the areas when I find them, because they are always different spots. I use special industrial urine cleaner. So it does take the smell away from him and I, but he just marks it again or somewhere else. I even try taking him to the spot kinda like you would with a dog that crapped in the house, and let him smell it and I tell him "bad cat". I even go as far as putting him in the glass door shower and leave him for a time out.

There is no stress around here that I can think of. We are a quiet couple and we love him and spoil him to death, with treats, the good $20 dollar litter
and the good Blue food. He has fresh water all the time.

I also have a full apartment, top - bottom, with my rare international collections. I can't have him spraying all over porous materials, that I can't use the cleaners on.

He once sprayed on my rare collector baseball cards. I had to wipe them all down and store them in a air tight bag with scented baking soda.

I can't put everything up at hip level so he doesn't mark it. That is ridiculous. I am at my wits end and I refuse to give him up.

Please help.

(Yes I know I have a lot of stuff, I like it that way.)