I'd try Feliway, at least.

Putting a cat outside because cats are moving into the area will make him worse not better...until something happens to him.

He is already outdoors by his own choice for almost the entire day. He could come inside using the cat door but chooses not to.

I don't understand the Feliway thing. Start spraying that around the inside of my house instead?

Kate

I thought my cats were marking, both of them.

I had caught each of them peeing in the house.

One day I noticed one of the cats using an empty, clean litter box to pee in. I started cleaning the litter boxes more thoroughly, and now I tip each box and give it a shake after cleaning, so one end is free of cat litter. It turns out both of them prefer to pee on the plastic, instead of squatting in the clay.

Since I started doing this (fingers crossed, her) neither cat has marked inside the house.

Feliway is based on an enzyme that cats secrete to let other cats know "we come in peace".

It'll calm your cat & hopefully he will stop thinking he needs to mark his territory.