Sick cat with nasal discharge. Help?

My cat started sneezing and getting nasal discharge mostly from the left nostril. Turned out he had three bad teeth on the left side that had to be extracted. Around the same time I had some water damage in the basement and in the kitchen. We had drywall replaced in both areas and new carpet put in. A lot of insulation fell out of the ceiling and created dust. Also around the same time I changed his food gradually from Iams (which he had eaten his whole life) to Blue Buffalo. Anyway it took three rounds of antibiotics to see any improvement with the nasal discharge. It never completely went away because he kept sneezing. Now the infection is back and is worse than ever. He has VERY noisy breathing and a ton of mucus mostly on the left side. He went in today and was put under so he could have a nasal flush. That went okay. The doctor also checked to see if had any any infection in the teeth from the previous extraction, but there wasn't. He just had A LOT of mucus backup in the left nostril. They are holding him overnight with an IV with medication and fluids and they are seeing how he responds to nasal antibiotics. They are also going to put him on oral antibiotics again. Do you think the symptoms are caused by the bad teeth, the food, the dust, or is it something else? The doctor wants to do an x-ray if the infection comes back again after this next round of antibiotics. I'm so worried. Does anyone have any experience with this or advice?