What did the culture say? Did the vet change antibiotics?

Also, running the hot water in the shower to create steam will help him feel better. The plastic container w/ water and a lid does the same thing though when placed over heat. When I was having to soak one of my rescue babies because of an RI, I would leave her for a good hour. She'd come out hydrated and breathing alot better.

Keep him well hydrated though or his kidneys will shut down. The antibiotics wreck havoc on their kidneys, so keep flushing the water through and make sure he's urinating on a regular basis.