Thank you for keeping us informed. I hope the little fellow is starting to settle down now, and is starting to use the litter box. I hope his urine flow is getting back to normal.
It is quite surprising that the vet didn't do a urine culture to determine bacteria count before prescribing antibiotics. I assume the way you speak that she mustn't have done even a basic urine culture. She had the animal for 7 days I would think she would've been able to have collected a urine sample in that time and done a basic urine test to be able to give you some ideas what the problem was. I do the basic urinalysis test at home on my 5 cats including microscopic and specific gravity, and if I get a hint of any problem, it's off to the vets. I think if I were you I would be going after the vet to get reports on what had been done, and start a health file for him in case they might be needed somewhere down the road by another vet if something else arises. All 5 of our cats have their own files in my drawer.
I would have to go along with pattymac on trying to get the little guy onto wet food whether it be canned or raw it's a far cry from the dry kibble, and so much healthier, and it's getting more moisture into the body. Something else might want to think about is a water fountain. I use one with my cats and find they consume more water from it than they do from a bowl which helps to reduce the chance of urinary tract trouble.
As far as getting another vet's opinion, that's your call. I think if it was me, I would be giving it serious consideration, and that's another reason for having the previous work records to show to a new vet, if you so do decide.
Animals are such agreeable Friends.
They ask no Questions. They pass no Criticisms.