The best thing to do if your dog won't stop sneezing is to take it to the vet. There are many causes for your dog's symptoms, and some of them can be serious.
Most sneezing in dogs is caused by the environment, such as allergies to pollen and smoke. However, there are some infections, such as a sinus infection, that can cause sneezing. A severe tooth infection may also cause sneezing if its draining into the nose. There could also be foreign body stuck inside your dog's nasal passages. Finally, sneezing can be caused by intranasal tumors. However, this is normally found in animals older than eight and is not very common.