That´s one expensive antibiotic! Recurring infections is a sign of
immune weakness, alkaline urine pH, and not enough urination. If he
were my cat, I would try to solve the problem with a high quality
canned diet (e.g. Wellness, Felidae, Wysong etc.), give him a raw
snack daily (beef or chicken), give him raw liver once a week and
supplement his diet with B vitamins. I would only resort to abx if
this didn´t work. Have you tried the online drugstores?