Ange, thanks for using our service.
Some cats make very quick improvements, some don't. You will usually see improvements within the first 5 days of treatment.
Your veterinarian may recommend another round of antibiotics tomorrow, this time it would be wise to try a different antibiotic such as Baytril. If a second round of antibiotics doesn't help, doing a culture and sensitivity on the urine will help. A C&S is basically a bacterial screening that tells the vet what type of bacteria is in the urine and what antibiotics work to kill it off. Generally, Clavamox does help but sometimes a different antibiotic can be more useful. You simply don't know without trying, in some cases, so don't feel like you're not making any headway. Sometimes this happens.
Keep monitoring eating, drinking, urinating and defecating. Speak with your vet tomorrow about sending home some A/D, which is a very high palatability food that most cats will eat without any problems (even those that are not feeling well or are picky). It's important for cats to eat consistently, their liver can become damaged if they go too long without food. You may have to look at force feeding him a few syringes of food for the first few days if he does not start eating on his own.
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Veterinary Nurse
8 yrs experience with animals, 3 in emergency med. Owner of 3 APBT and 3 sphynx