Is there any swell around his closed eye (especially compared to the other one)?
When the eye is openned, is it fully openned, and is the eye bright or cloudy/whiteish?
Sneezing / popping / barking / crabbing... while sleeping is not uncommon, in fact I have never experienced sneezing as a sign of illness with gliders, when mines had a respiratory infection, the main sign was a loud popping sound while awake and active, but never sneeze nor cough.
In any suspicion of eye / respiratory infection, it's always better to consult a vet anyway, who can prescribe antibiotics if needs be, and even if the glider turns out to have no issue, at least it serves to calm down the human which can only be positive for the animal (gliders do pick up on our stress and act in consequence) ;)