Birds can show indications of allergies. It is pollen season now so check to be sure all of your windows are closed. Suggest you get an air filtration system if this is the cause. Is the bird itching too? If the discharge changes to yellow or green then that is an indication of infection so a vet visit is in order. Watch for itching or lethargy. If the sneezing and discharge is persistent and bothering him then a vet visit is needed. Check in his nares to be sure there are no seed hulls inside or other irritants.