My cat has bad URI and antibiotics are not working. What can I do for him?

His URI is probably viral, and not bacterial, especially if Zithromax didn't work. Talk to your vet about two things - a decongestant drug for his nose, and also getting him on an immune system booster like L-lysine. Another possibility is he's got some sort of unusual infection that won't respond to either the Clavamox or Zythromax. In this case, a culture of the secretions in his nose could provide a clue.