Try spraying Feliway in the air. I use it all the time for my crew. It works great! It has an enzyme in there that only cats can detect. It's very calming. Try spraying the cat tree with it too and any bedding they may have. Do NOT spray it directly on the cats.

I have a shower pooper. My 20 lb tuxedo Spikerdude poops in the shower. Not all the time. But I've learned that it is a "marking" mechanism in a cat. Cat's anal glands cover the poo with the scent of the pooping cat. It tells the other cats that that is HIS crib! I've tried putting a litterbox in my shower for him. It just got too messy so I decided to allow him to poop there. I can scrub it down with bleach, and he hasn't done it anywhere else. Poor cat was dumped at a shelter in a cat carrier 3 sizes too small. He's a great cat, except for that one little "indiscretion".

Thanks for not giving up on him. I'm sorry I didn't answer this before. Been busy with a dog stabbing trial. Get some Feliway concentrated spray. It really works!!!