Thanks everyone.
Just so you guys know how fast this became an issue- last week he was at 3 different vets.
Monday 9/22- Took him to his regular vet for a check-up because we just got him (she even did a chest x-ray because he was throwing up). Everything was fine. She did mention it was difficult to hear because his breathing is very loud.
Tuesday 9/23- The next day, we had an appointment at the hospital he's in now, with the vet that was supposed to do his neuter and palate/nare surgery. She also checked him from head to toe, listened to his heart and lungs, and had a consultation about his patella luxation (soft tissue/ortho surgery her specialties and I trust her 100%).
She said she "thinks" everything's fine but it's difficult to tell over his breathing. He's very, very loud.
Thursday 9/25- Two days later we took him to a vet a few blocks away, because I thought he had fleas (false alarm). She checked him from head to toe as well and also listened to his lungs. Again, everything was fine.
10/4 - He's being hospitalized for aspiration pneumonia. I'm baffled. How quickly does this develop??
Three vets, no one was able to hear or see anything.
Just the worst luck.