Where do you live?

You can't keep him in your bathroom until he is well? I'm sure quite a few of us have antibiotics on hand that we can send you. That, combined with steam from your showers/baths, and good food might help him pull through the URI.

Did you put food out for him, and water? Can you insulate the carrier by wrapping newspaper and plastic around it? Or put the carrier in a large box that is wrapped with plastic. If he is going to have to stay outside without help, it may be more humane to call in the authorities to shoot him, better than a lingering death from starvation because he can't breathe well enough to eat.