I just made another appointment for him and I'm definitely going to bring up the possibility of doing a culture via respiratory flush (if it's even possible on a snake his size, he's about as small as full grown males tend to come). When he got the swab culture done there was a bit of blood in the sample which the vet said might mess with the results, but in the end I was told the sample turned out to be fine. I've always wondered if it ended up being a bad reading.
I would be less concerned if his symptoms were holding steady and it just seemed like something chronic but the recent sneezing up of white fluid is something new so that's what has me worried right now. I'm seeing a different, very experienced, vet this time to get an overall second opinion on next steps. Fingers crossed!