I don't want to discourage you further, but we fought the UTI monster on and off for about a year.
We also used C/D food. It kept Beandip's pH where it needed to be, however he still relapsed almost monthly (6-8 weeks on average) despite that. We ended up putting him on Prednisone for inflammation. It has long term risks but the benefits have been good. Diet + the steroid wasn't enough...we are covering the anxiety angle with Prozac. I'm elimitating dry food altogether now, and I believe that is making a big difference. I also have water ALL OVER the house, I mean cups and bowls in at least 10 places.
I didn't mean to scare you. I don't think your boy needs all of those other drugs just yet....sorry. But diet and water intake are critical.
We haven't needed any antibiotics for almost 5 months now! My vet prefers Baytril (antibiotic) for UTI's...I think Baytril might be somewhat similar to Zenequin, though. She said things like Clavamox are hard to "get into" the bladder...not literally, but I think you know what I mean. Clavamox is better for respiratory infections.
How long have the courses of antibiotics been? He might need a longer course...like at least 14 days if not more. I would ask about a follow up urinalysis too, after the antibiotics are finished. Relasping after 10 days makes me think the bacteria wasn't completely eliminated. A truly bacterial urinary infection is actually fairly rare in a young cat. It's more common in humans, dogs, and older cats. But it does sound like bacteria in your boy's case.
How old is Simba? I can't remember.