Seems like you have the husbandry in order and your boy is eating well. That is great right there !
Were those vets actual reptile experienced vets ? Has any of them ever run a culture?? That is so important !! There are different kinds of causes for URI and some have need a different treatment then others. You need to know what is causing the problem. Virus? Bacteria? Secondary infections?
Usually a lung wash/culture is done and then the snake is put on a broad spectrum antibiotic to begin treatment. Once the culture comes back, you may have to switch to different antibiotics because the germs may be resistant to the one you use. There could also be a fungus in the lungs, which requires a anti fungal med. Without knowing what is going on in there, treatment is a stab in the dark.
Also, treatment sometimes has to be given for a couple of month, rather then just a couple of weeks. What meds were you given? How long was treatment ?
Most likely the meds you got started to help "some" but weren't enough or the right ones, and the problem returns. Sometimes worse then before.
That is why it is so very important to have a very experienced reptile vet when you take a snake to the vet.
Reply With Quote