I'm not seeing anything (old shed pieces) in his nostrils...are you? Have you looked inside his mouth? I wonder how long he's had these symptoms...did you get any info from whomever you got him from? His face looks a little smushed-in...some snakes get a mouth infection after hitting the glass (often being teased or just over-eager about food). I can't tell from here what's going on...vet seems like a good plan, there's only so much we can do online to sort this out. Most infections respond better when treated promptly, & after seeing him close up, that appears to me to be what's going on. (short answer: sooner)