Aw man, bummer !!
Honestly, Dave. If you take him to the Vet again, why not run that test. It only takes a minute to swab and at least you can rule things out. It seems odd that Yafe is not responding to treatment when his URI wasn't even that advanced to begin with.
Perhaps its just me, but I would like to rule out some issues that tend to cause sickness that doesn't respond to treatment.
That said, I still hope he will finally just turn the corner for you. This has been so stressful on you all, incl. Yafe. You are in my thoughts..
edited to add: Perhaps another culture as well ? Just to see if any of the bacteria (and which) has been eradicated (or not). And what else may be brewing. Might as well, if you do end up taking him...