Sparky went through this in the fall for a couple of months and for part of that time his nose was running (just clear liquid). I didn't know of anything that changed and it went away as quickly as it started..
We did anti-histamines, then antibiotics. None seemed to help, but just as I was getting ready to call the vet to schedule a rhinoscopy, I noticed he'd stopped sneezing.
The vet told me dogs can develop nasal allergies/irritation (rhinitis), although it is not very common. I guess that is what it was for us. But you could have a bit of fox-tail, or a plant in his passages so you probably will want to have the vet look at it. I think your vet will want to examine first and treat conservatively before the anesthesia and rhinoscopy and I'd book a specialist for that if it comes to it.
Good luck & keep us posted.
(a.k.a. Lucky's & Sparky's mom!)