glad you got him to the vet and got some antibiotics. i assume it was for a UTI. i don't know how long it takes for it to really kick in, but i'd ask the vet about some pain meds too maybe. sounds like it's still pretty painful for him to urinate and that's why he's hissing.
i'd probably keep trying to feed him the hills for urinary tract health in case he's going to be prone to UTIs in the future. maybe you could mix some other tastier wet food with it, though, to encourage him to eat it again? then hopefully you can wean him off the regular stuff and slowly replace it with more and more of the hills. i've read a bunch of other stuff on the urinary health diet and you COULD try some of the suggestions, but i hate the idea of risking another UTI by going off the hills, too.
i don't have a lot of experience with cats prone to UTIs but i did foster one who luckily did eat the stuff without too much of a problem. that is until i was given a couple more to foster at the same time who DIDN'T need the urinary tract diet. as soon as she smelled the normal wet food, she wanted nothing to do with the urinary health food. once she went back to the shelter to be adopted, though, she ate it again just fine because she had no choice.
hope your boy is back to fighting condition very soon! you too!
Kitty, Lucy, and my sweet Angel forever in my heart.