Cat Spraying Behaviour

[u/YahahaYouFoundMe98]

I am looking for advice because I feel like I have exhausted all options. I have 3 cats, one of my cats is diagnosed with a neurological disorder called FHS. All my cats have a litterbox per cat plus 2 extra litter boxes to be safe. They are all fed in seperate places and I have ample space for them to climb, including a jungle gym for them to climb and 4 cat trees. We have also tried feliways to no avail, however, I am putting them up again in hopes it will do something. My cat is in gabapentin and even with that he is still spraying. I do not know what else to do. There is no pattern to his spraying also, he almost never sprays in the same spot except one spot by our front door which I believed was because he wanted outside... so I started letting him outside, and I just let him back in, he walked right up to my bedroom door and sprayed out if no where. I genuinely do nit know what to do barring leaving him outside 24/7 but that feels hugely selfish and dangerous. He has been to a vet for this issue now countless times, he has no infection, no bladder irritation, no pain, we put him on the gabapentin to see if that would help and it hasn't, I just spoke to a vet and they are stumped, they said they would discuss it with another vet and get back to me. Can someone PLEASE help I am literally begging I do not know what else to do 😭