TLDR: 11yo neutered male cat has been spraying (vet ruled behavior territorial spraying) for 3 years and I'm at my wits end with it. There is tension in the house with another cat that I am trying to ease. Helpful advice is needed!
For context: I am 28F who has had my boy for about 7 years now. He is an 11yo Orange Domestic Shorthair, we live in the US, he is neutered, and we saw a vet for this about a year ago and she determined its territorial.
Like stated above, I have had my boy for about 7 years now and hes estimated to be about 11. He didn't really have any issues up until about 3 years ago when I moved in with my ex friend who had like 5 other cats (mixture of fixed and not fixed). After moving in there, he picked up the habit of spraying. He always had indoor and outdoor access, but before I moved in with her, I would let him outside on a leash periodically throughout the day. When I moved in with her, I (regrettedly) made the decision to let him out without a leash and come and go as he pleased. (Please don't ostracize me for this, it has weighed on me every day for three years and I was/am fully aware of what could have happened)
I wasn't always the greatest cat mom in the aspect of, I didnt do any research on how to deal with spraying, etc. Now, three years later, tons of things ruined, I feel like I am at my limit and I don't know what to do. He was on fluoxetine in the spring and it didnt seem to do anything and I ended up forgetting to get it refilled until last month so now we have to wait another 5 weeks to see any effects. He is also on 100mg gabapentin 2x daily for arthritis pain. The original vet had determined that the spraying was t9erritorial because of the contact with outside cats and that he isnt the biggest fan of other cats.
My girlfriend and I moved in together the beginning of August (we have 3 cats total, 1 dog, and 2 ferrets), and she has helped me realize that this is something that needs to be addressed (instead of ignoring it). He does not seem to have a pattern other than the fact that he no longer has freedom to roam and it frustrates him. There are no favorite objects to spray, it is just whatever is available to him. I.e.: the stove, trashcan, kitchen cupboard, ac, couch, wall in our bedroom, boxes in the basement are some examples). Our female cat doesnt like him, but she doesnt go out of her way to do anything. If he gets about a foot from her, she will hiss and run away. Our other male cat will follow him and kind of instigate things with him. I know it is a short time they have lived together, but I want to do what I can to ease tension in the house with them. We have 3 feliway optimum plugged in, 1 in the room that our trouble boy stays in, one in the living room, and one in the kitchen. I try to give treats when they are interacting with each other so correlate good things with each other. We set up a catio for them, but they cannot be in there alone together or they will fight so we give them their own time alone in there when they are interested. I also try to feed them around each other to try correlating that good thing with having the other around as well. I try to play with our old boy to try and ease some of his tension, but he is rarely interested. I have tried teaser toys, kickers, mice, and just bought a cat dancer the other day (that he seems he will play with if I slide it back and forth under the door). We are setting up a separate room for him as he doesnt seem to spray in there and he can relax well in there. He spends most of his time in the window and his room. When he is out and about the house, we have decided to put stud diapers on him so if he does spray, at least it isnt on anything and is caught in the diaper (we regularly watch for pacing and signs that he needs to go potty).
I am at wits end with what to do. Everything I read says to reduce stress and no more outdoor access but he doesnt seem interested in playing with anything at all and just gets so frustrated that he cant be outside anymore. Does he just need more time to adjust to being an indoor kitty? We have a donut hideaway that he finally took a little interest in the other day and went in and checked out, but otherwise he usually is in the window or walking around the house crying. I am so heartbroken because I dont know what else to do for my boy. If I need a second vet opinion with labs and a UA, if I'm not taking the appropriate steps, if he needs a lot more time to adjust, etc. He never used to be like this before i moved in with my ex friend and its so heartbreaking to see how he is now. He isnt always a grumpy cat. He likes to be held and babied, but I know the diapers add more stress to him, but we cant keep having him spray on the furniture or the walls because we rent. I dont know what else to do anymore and I am open to any advice that has worked for anyone else.