Chronic reoccurring urinary crystals

My male cat has had chronic reoccurring urinary crystals and uti’s. Sometimes as often as once a month. He is on prescription wet food/ treats (Purina urinary, has also previously tried hill’s science) and Prozac. The Prozac is because the vet said it is likely stress related. The vet knows us very well at this point and call him the problem child lol. They say I’m literally doing everything I can. I’ve read somewhere that I high protein food might help, but I’m not sure. I am really at a loss. He only shows symptoms of a flare up when he is hurting so bad he can’t keep food down. Any advice for anyone else having this problem would be great. I am really considering switching his food to something else even if it’s still a prescription kind. I’ve also recently found out the Purina is owned by nestle so that makes me want to switch more. Thanks in advance!