Birds sneeze with weather change, moults, irritation if the air is too dry, dust, humidity, illness.....
Lots of different reasons! If it is too dry, I hear humidifiers are great. (I'm in Britain, it's never too dry!) If it's a moult, it's quite normal (as is "picking the nose" lol). If it doesn't clear up or goes on for a long period of time then obviously it's time for an AV.
If your bird is moulting, giving him a bath or a spray with warm water will help.
Additionally, there can be other causes such as a certain perfume/aftershave, washing powder.....
I found Cal was sneezing when she first went into the lounge when the other pets had been in there so I vaccum a lot every day. My hoover is a pet specific one that gets up all the doggy, rabbit and cat hairs too which is useful, but also it is a hepa filter which helps with human allergies to dust.
Many things to whittle down but most commonly it is a simple solution. The fact there is no discharge or obvious discomfort indicates to me it's one of the pesky small things!!! Enjoy your quest!
