"Deuteros" wrote:.

I don't know what else to do and if he doesn't stop I'll have no
choice but to put him outside.

I had a cat that did this. The only thing that worked was to confine her in
the utility room with her box, bed, toys, and food and water, and go see her
and praise her and give her a lot of attention from time to time. After a
couple of days, I let her out. If she "eliminated inappropriately" again,
back in she went. She stopped after three-four times being confined. And she
stopped for years.