Piet

Occasional sneezing is normal in birds, much like us. Frequency and discharge are what change the picture.

Night song on its own is not a health problem, so the reassurance in this thread is fine. The things that would change my mind are in the day, not the night: fluffed up and quiet when he should be busy, tail bobbing with each breath, any noise in his breathing, droppings or appetite shifting, weight coming off.

You say he is fine in the day. Then treat this as a light and routine problem and shorten his day. Long days push hormones up, and a hormonal male sings at whatever hour he likes.