Handfed cockatiels, especially males, do not always make good breeders. They are definitely not bonded as a pair so it is far too early for them to have a nest box. You need to remove that or her aggression could turn into serious attacks. If you want them to breed, you can’t handle the male anymore. You need to give the birds privacy and the chance to bond without human interaction. You should only be around them to give them food and water and clean the cage. Otherwise he is wanting to be with you rather than the female. If you still want him for a pet, I would split the pair up and get a wild mate for her. Also, if he is not at least 18 months old, then he is really too young to breed. The female should be at least 2 years old. If you still want to breed them rather than have him as a pet, then you have to give them time to bond. Do not give them a nest box until they are clearly bonded – feeding each other, sitting together, grooming each other and of course mating.
Thank you for asking Lafeber,