We went through this with my boy kitty and it was no fun :(.
What we found works (knock on wood!) is grain-free wet food only, with extra water mixed in. We give him Wellness brand, which is high-quality and relatively low cost-per-oz on amazon. We also have a very quiet circulating fountain – brand is pioneer pet, it’s stainless steel so does not attract bacteria growth and doesn’t make sloshy/mechanical noises. If your cat won’t do a fountain, consider leaving a toilet lid up (many cats prefer the cooler, fresher water of the toilet, go figure). If you can only do the dry prescription food, try pouring warm water over it and letting it soak before feeding it to your cat.
If he has no blocking but blood in his urine, it is probably the crystals. Purina ST/OX food or a similar FLUTD food will help those along. Your vet should have tested him for crystals (this is a very quick test, they just look at them under a microscope). Your vet should also have give him an anti-spasmodic and an antibiotic (and I would ask for a painkiller, like Buphex, if your cat is crying/in pain). If he is also stressed, you can try a feliway diffuser as well. If you are having trouble pilling him, mix it with a small amount of food or low-salt tuna water. If he is not eating well, ask your vet if there is an alternate/compound formulation you can use – some feline medications can be given as a patch, a lickable paste, or rubbed on kitty’s ears. For antibiotics, ask your vet if a convenia injection would be appropriate – this was a godsend when our older cat had dental surgery, as she gets super stressed about anything going near her mouth.
What kind of litter do you use/how often do you scoop the box/how many boxes and cats do you have? Especially in the weeks following and your cat’s recovery, you want the box to be as enticing as possible to him, meaning clean, easy to dig/cover in, private, etc. I would also wash anything and anywhere he peed on with nature’s miracle or a similar urine-erasing enzyme cleaner. Also keep an eye on the litter box – I use clumping litter so we can tell if he is producing large enough urine clumps.
Hang in there! I remember when we had to leave our guy at the pet hospital overnight; driving away and wondering if he would make it through the day and the night was the worst feeling ever. But he did get better and his FLUTD is 100% manageable and he was back to his old self in a week after his urine crystals cleared.
Also, I would maybe look for a new vet. Sorry, but FLUTD/UTIs especially in male cats needs to be treated rapidly and aggresively. I can’t believe your vet just said “he’s stressed” and gave him prozac without testing his urine! And didn’t give you an anti-spasmodic or painkiller, or an antibiotic. That’s like if you or I went to the doctor for a UTI and came back with paxil! Yes, stress can trigger FLUTD flareups (and don’t beat yourself up about this, cats get stressed about weird things like changes in temperature) but UTIs in male cats can very rapidly jepardize an animal’s life, and are so simple to test/diagnose/treat.