Are you sure it is a classic sneeze? Many dogs do what is called a "reverse sneeze". I am not sure if that is the technical term but my dogs do it a lot. Check to see if the dog is breathing air "in" or "out" ... if in it is the reverse sneeze and it is harmless and sounds much more horrific than it really is.
In all of my years of being on dogsites and convos that have been had about it, I have yet to hear if anyone knows the cause. Even my vet said the cause of it is unknown, but it is probably just from an irritant near the dog at that time.
I would not panic about this if only the reverse sneeze. How often is it happening?
If this is indeed a regular sneeze...has the dog been boarded lately or been around other dogs? Dogs can catch a "cold" as well and the sneezing of another dog would render the germ airborne and your dog could catch it. If he is older or defenses down in general it would be more suspectible.
Your vet will give you the most accurate info. The dog could even be allergic to something in your house (have you used any new cleaning products? I used a new carpet cleaner once and my dogs both developed allergic reactions in the form of a rash on their chests..they are so low to the ground the product was causing irritation).
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"Be more concerned with your character than with your reputation. Your character is what you really are while your reputation is merely what others think you are." -Dale Carnegie