Hi,
Your cockatoo is much too old to only be handfeeding. Most cockatoos are fully weaned between 12-14 weeks. You should have started to introduce him to soft foods once he was partly feathered and starting to pick at things around him. At this point, you need to take him to an Avian Vet to rule out a medical cause for him not being weaned. There are viruses that cockatoos can have as well as other issues. If he has only eaten formula to this point, I would be worried about fatty liver disease, as the formula is not intended to be the sole diet for a cockatoo this old.
As for weaning, once you can determine he is healthy, you need to offer him a variety of foods before you hand feed his formula – this is for every feeding. You can cook brown rice and add mixed veggies and offer this warm. Also offer pellets, some fruit, as well as some soaked pellets. And fresh water. Put all of this in his cage and leave the room for at least an hour. He should not be able to hear or see you or he won’t explore the foods. You continue this before each feeding until he is eating more and more on his own, and soon he will reject the formula. He may be rejecting it now because of his age, but he may be sick in some way. It’s not normal for a 5 month old cockatoo to have never eaten solid foods unless you simply have never offered any to him. It is unfortunate that someone sold you this bird before it was weaned, and then didn’t give you proper directions on how to wean him. This makes me suspicious of the breeder and concerned about the health of this bird.
Thank you for asking Lafeber,
Brenda