Neutered Cat Spraying in the House?

I have 4 male cats (all from the same litter). They have lived together for their entire lives. They are now 8 years old. One of them sprays on everything. He even does it on the extra pillows on the couch (they will be leaning against the armrest, and he'll spray a spot and it will dribble down). He sprays on backpacks, plastic bags, and furniture. He's been doing it for a few years now. We've tried everything.

We have feliway in two rooms in the house (super expensive stuff so we could only afford 2 diffusers), we use a good enzyme killer on the things he sprays, we have tried having one on one interaction with him, we have 6 cat trees of varying sizes. We clean the litter boxes daily. He's been to the vet and they recommended feliway and this calming food. We've tried both. It seems like the calming food has caused him to pee more. We've tried keeping him locked up with his brothers whenever we have company to keep him relaxed, but that usually ends up with him spraying on something. It happens instantly. We left the door open to a room he isn't allowed in, and within minutes he had sprayed on something in there. We don't ignore him, we don't punish him, we don't rub his nose in it. We clean it up and spray feliway all around, and still he continues to do it. What else can we do?