I've hesitated to post because I'm not a vet or anything, but I do have quite a bit of experience with UTIs and FLUTD. Do you have a definitive diagnosis for Mokkun yet? Do you know for certain he has crystals, or ?
One of my cats did have a definitive diagnosis, first of UTI and later of crystals and ended up having PU surgery. He was on a Science Prescription kibble diet for most of his life, first c/d and then later w/d because the c/d made him gain too much weight. In retrospect, I'm not sure the Science Prescription was necessarily the best long term for him. Not long after he came home from surgery, he developed a voracious hunger and I suspect based on what I've read about feline diets that the c/d may have been nutritionally deficient and causing this behavior. If only I had known then what I know now, I'd have put him on at best a raw diet or at the very least a canned or homemade cooked diet. I'm not convinced he was ever crystal-free being on the c/d or w/d anyway but at the time I was afraid of not doing what the vet instructed.
My ragdoll has also been diagnosed as having FLUTD, but in his case, he has no crystals, he has no UTI, and the regular vets (he's been to 4 different ones) tell me "some cats are just like that." So we went further with diagnosing him with my favorite but super-expensive vet and have figured out that his bladder becomes inflamed when he's stressed, resulting in UTI symptoms (but with no UTI present). What I've also figured out is that if he eats any kibble--any kibble at all--his symptoms will appear and slowly worsen. So what I've been doing for him is 1. attempting to keep him as calm and unstressed as possible, and 2. not allowing him any kibble at all. His diet is primarily raw, with the occasional can of Weruva (or other high quality canned food). We also have a water fountain and multiple litter boxes (2 per floor). To minimize his stress I run feliway diffusers almost constantly and when he starts to get cranky, give him Spirit Essences (also called flower essences) UR Fine and Grouch Remedy. I know people doubt the effectiveness of spirit essences, but it really, truly seems to help make him less grouchy or stressed.
Anyway, all that to say that I think if you don't have a 100% definitive diagnosis, that is probably the first thing you need for him. Urinalysis, bloodwork, ultrasound....it's expensive, but a sick cat and PU surgery is much more expensive.
HTH.