Cat peeing outside of litter box

Hi there! I have a 9 year old neutered male cat that I have had since he was a kitten. I can’t remember when he started peeing outside of his litter box but it was pretty early on. I had two litter boxes at one point and only one was being used so I got rid of the second one. I do have another female cat (spayed) that is around the same age. The two get together well and love eachother. I have taken my cat into the vet to ensure there’s no medical reason for his spraying (I used to work at a vet clinic and have had multiple blood panels and urine samples done on him) and have tried around with different litters, trying to keep up with keeping the litter box clean to his liking (I work long shifts so it’s hard sometimes) I’ve changed out his litter boxes to bigger ones as well. I’m onto some last resorts now, I just purchased an extra soft litter, second litter box again just to try and a pheromone diffuser. Hoping those will help but was looking to see if anything else has worked or helped anyone else besides putting him on a daily medication for anxiety (he’s old and has a weakened immune system due to feline herpes). I love him to death and I hate having to discipline him for every time he pees outside of the box and deal with cleaning it up all around the house in seemingly new places often and on personal belongings. I got him a new cat bed/tunnel recently and within 2 weeks he peed in it even though he had been enjoying using it. I seem to believe it’s a territorial thing and that he is very OCD about his litter box.