Parrots have very sensitive respiratory systems and like humans, sneeze when something "gets up their nose" or care, in their case!
There are many different reasons why your bird may sneeze and I will list some of the more common ones below along with possible remedies.
Air fresheners are very dangerous to birds due to their sensitivity. These should be avoided at all costs. Scented candles can also cause respiratory problems and sneezing. Instead, try to fragrance your home with some fresh flowers, mint leaves or a small dab of essential oils on a cotton pad behind a radiator. I personally make my own "freshener" by putting fragrant flower petals such as rose and mint leaves into some distilled water and spraying it on my curtains.*WARNING* May attract parrot to curtains LOL
Furry pet hair
Just like humans, birds can find pet hair uncomfortable and if it reaches high levels then the sneezing will commence! Vaccumn as often as you can.
A change in the season
Wet to dry, dry to wet....your birds nostrils could be affected by either! At the turn of the season it is quite normal to find your bird with a nail up it's nostril itching away! If the weather is warm and the air is dry, try taking him into the bathroom with you while you bath or shower to give him some humidity. Regular baths will help also. The same is true for the reverse and a dehumidifier can work wonders!
Again, dust will get right up your birds care if he is sensitive! Birds create their own dust also, especially birds such as the African Grey. When your bird molts, the sheaths that contain his new feathers will be disintegrating and causing dust continuously. Vacuum and dust as often as you can, air your home and quickly wipe over your birds cage daily with a damp cloth to help alleviate this. Air purifiers can be bought to help filter the air.
As I have said, there are many causes for sneezing and only you can be observant in your home to get to the source!
If there is nothing obvious and your bird is fluffed up, losing balance, has unusual droppings, nasal discharge or isn't eating or drinking, then you must get him to an avian vet immediately.
It is worth remembering that we all sneeze and as long as your bird is perfectly normal in every other way, then this should pass as quickly as it came on! It is also normal to find your bird releasing a small amount of CLEAR fluid from time to time during a sneeze is nothing to worry about either!