I'm sorry this is happening again! I know the frustration!

How long has he been on the S/D food? It's not meant to be eaten long term. It's not a complete food, too low protein. Pete hated it, but the longest he's ever had to have it was two weeks.

Pete was just switched to Royal Canin S/O. Vet said that he has way too many crystals for eating C/D for over 4 years. S/O is safe for all the cats so I just switched them all. Fortunately, Pete even likes the canned formula! He has to be retested in a month to make sure this food is working better.

Is there actual infection along with the crystals? The anti-biotics won't do diddly for the crystals. If there isn't infection he shouldn't be on them. Since he's had them so often, he may be developing a resistance and you might need to switch types. Pete gets Baytril for his infections most often.

Has your vet suggested other medications yet? Pete took amitriptylene for awhile. It's a human drug that actually does a range of things--mild sedative, mild anti-depressant, used in the treatment of chronic pain. It's used in cats if they think the problems are stress related. Prozac is suggested sometimes too(for the cat, not the owner!).

Do you have an empty litter box? Since he seems to like hard surfaces, he may like an empty box. Pete did for awhile and still likes the box that has very little litter in it. You have to clean it alot, but it's better than cleaning your cupboards! I also have a sssssssssCat canister on my kitchen cupboards to keep someone from peeing on them, but I suspect that's Onyx not Pete. (Onyx has behavioral, not health issues)

Have they done an ultrasound of his bladder? If he has calcium crystrals or stones, they'll show up on an ultrasound. Pete had one--his bladder was basically full of sand at the time (before his surgery) and he has thickened bladder walls. There is a surgery that goes in and removes the build up from the bladder walls.

If you trust your vet, I'd stick with them. IFLUTD is extremely difficult to treat and you aren't likely to get any better results with a different vet. If you have a vet school or a specialist clinic near you, a consult might be good though.

Good luck! I hope you find something that works for Pouncer.