So, the vet put him under, looked up both nostrils, and found nothing up the boy's nose. Nada. No foxtail, not even mucous membrane damage to indicate that a foxtail had been there. What could it be? According to the vet:
1.) Could be an allergy of some kind.
2.) Could be a foxtail that has somehow been snorted wayyyyyy up there, only detectable by the use of a pediatric endoscope through the mouth and up the back way into his nasal workings. That's apparently specialist domain; regular vets don't typically have to do this.
3.) Could be something else. (Thanks, that was helpful.)
At any rate, I was instructed to keep an eye on him for further sneezing episodes, which would apparently get worse; excessive discharge, in colors that would indicate the presence of infection; and/or blood, which is just never what you want to see in any case.
So today I went downstairs for a couple of minutes to get something out of the truck, and when I got back up to the apartment, my roommate and I had the following exchange:
Amy: "Um, Truman was making this awful gagging/choking noise, and then he spat something on the floor."
Me: "What was it?"
Amy (holding open a napkin): "This."
Me (freaking out, after a split-second look at what was once a spider): "EWW! You could have just said! You didn't have to SHOW me!"
It was a few minutes before it dawned on me that the spider could have been our option number three. Makes sense, in a way -- no mucous membrane damage, possibly bit him in the mouth or nose and caused irritation that might result in sneezing. Also makes sense that Truman would find a way to present me with some kind of horrible nightmare starring creatures that freak me out more than just about anything else.
He is still sneezing occasionally, but it's just once or twice instead of the crazy, minute-long violent sneezing jags he had on Wednesday and Thursday. No funny-colored dog snot, no blood.
My dog the spider-snorter. Figures.