It could be hormonal behavior as the Cockatoo hormones are raging this time of year. You didn't mention what his normal behavior is so just guessing. It could be he is sick. Birds hide illnesses well otherwise in the wild they become meals for bigger birds. Is his poop normal, is he eating and drinking normally?
Our Too often spends a great amount of time in her cage during mating season. She just finished this years season so is back to normal.