My Cat Has Suddenly Stopped Using the Litter Box

[Chardonnay]

Hello! I'm new here, so if this topic is posted in the wrong area, please let me know. I'm joining every forum I can find for some help!

Just a smidge of history: I've had cats as far back as I can remember, and am quite knowledgeable with animal behavior. I currently have three strictly indoor cats and a Pomeranian.

One of my cats (a male) has stopped using the litter box(es) altogether. He was a rescue, he's about 6, and I adopted him when he was about nine months. He gets along just fine with my other pets. In fact, he prefers them over me. No, I haven't changed the brand of litter. Yes, the litter box(es) are cleaned every day. No, there's not a new pet in the house. No, my routine hasn't changed. No, I haven't moved recently. No, there are no health issues. I have gone through all the theories that I know of to try to figure out what is causing this and how to correct it.

He urinates and/or defecates wherever he pleases. Mostly on my bed or my couch (whether I'm on them or not). I had a delightful discovery last week when I moved a chair away from the wall to vacuum and found a pile of mess.

I don't know what to do! I would never give up a pet due to behavior issues. I'm in this for the long haul no matter what. Can anyone suggest anything to get him back to the litter box? I don't want to keep him closed off from the rest of the house, but cleaning up after him is quite a chore.

Someone suggested Rescue Remedy, Feliway or Comfort Zone and the like. Has anyone tried these products specifically to get your cat to start using the litter box again?

Any advice is appreciated!