Here's another thing I really want to mention. As I am sure you know, UTIs can be painful. When you said that last night he was lying on the bathroom floor and his head and paw were jerking. This, to me, means he was in pain. When I have had cats with particularly bad UTIs that were painful (crying in the litter box, and signs like Panzer has had) my vet has given me Buprenorphine for pain. It made the cat WAY more comfortable. .... but now that I am thinking about this more... didn't you say that you thought there was no way you could get it into him? They could do an injection of Buprenex at the vet, but it won't be very long acting (usually 24 hours, although there is a newer, longer lasting shot called Bup SR that lasts for, typically, 48 hours.) Maybe 24 hours will get him over the hump as the Doxy starts to work (it just HAS to work!!)