I know this is a late reply so hopefully he is feeling better... But if you notice any side sucking, deep breathing, or open mouth "breathing" that is a bad sign... and he needs emergency care. Rats breathe through their noses, so a tell tale sign of URI is a snuffling/chuffing sound and/or porphyrin around the nose. NOT to alarm you, but we had what you described happen to one of our dear girls and she sadly passed away... after seeing two different vets with conflicting advice (heart issue vs. respiratory). I think she had an undetected heart issue... but it's impossible to know. Mycoplasma/URI however is MOST often the cause, and it's symptoms range all over the place... so it's hard to pin it down sometimes. Doxycycline and Baytril combo typically works best for it, and side effects are rare... as long as you're getting vet prescribed/properly mixed and dosed meds. I would definitely try them ASAP if you haven't, and he isn't all better. I've seen more side effects relating to intracranial pressure relating to Doxy than anything, so if Pituitary Tumor was suspected that is a reason to pause... otherwise I wouldn't hesitate to use them. I have one girl on them now for what seems to be a mild URI/myco flareup. BEST of luck to you and him!