How is he acting?
I too vote for switching to a new bottle of insulin to remove that variable from the equation. I’ve had bottles of Prozinc lose their efficacy and feel like I was shooting water; other vials were good to the last drop. It’s an easy change to make. I’m glad you’ll be testing and ready with your hypo toolkit given that, if the insulin was bad, your prior BG tests aren’t particularly “telling” and it’s as if you’re actually starting out at a higher dose.
As has been mentioned, that’s not necesssrioy true. Clavamox is frequently used for UTIs and has an amoxicillin component, but I’m not sure amoxi alone is the first choice of most vets. A sterile sample of urine drawn by cystocentesis (needle directly to the bladder) followed by a culture/sensitivity is the only way to know for sure. Antibiotics can skew the results, however, so you’d need to wait at least a week after stopping antibiotics to get a sample. Just something to keep in mind.