This may be caused by something getting into the nose, like pepper, or it could be a symptom of a viral infection that is causing inflammation. If your dog also has a runny nose it is likely that he is going down with some kind of virus.
An infection of the sinuses is another possible cause of repeated sneezing. Sometimes this can be caused by a tooth infection. An abscess in the upper jaw can easily spread to the sinuses. In this situation you will probably realize that the dog is in pain and needs treatment.