My cat won’t pee but it’s not a blockage. We don’t know what to do

My cat (male, desexed, 4yo) stopped peeing so we took him to the vet and they gave him a catheter. They tested his urine and blood and everything came back normal. When they’d emptied his bladder they sent us home with a new urinary diet, some medications, and advice to monitor him and contact them if he wasn’t peeing. Over the next week he only peed twice. We brought him back to the vet a few times and called them basically everyday for advice. He seemed to have no physical blockage that they could find. If the put him under they can manually express his bladder but he won’t pee on his own, he doesn’t even try. He has no interest in the litter box. According to them he’s perfectly healthily in every other way, eating and drinking completely normal. It’s like his muscles are too tight to let him pee so only under sedation or muscle relaxants can they get anything out. After some frantic googling I found a few reddit posts and articles talking about urine retention in cats on Fluoxetine. I suggested this to the vet and she’d never heard of it but wanted to do some test so we brought him in. They’re not sure if this is causing it but they’ve started weening him off of it an in the meantime he’s in the vets so they can get him to pee on his own. They’re still not sure that it’s the Fluoxetine and if it’s not they have no idea what to do. Please has anyone experienced anything like this before and do you have answers? I just want him better