Thank you!Hi there, in this thread you'll find some helpful info about diet.
In short, the best diet for your cockatiel is variety. A good quality seed mix with varying types of seed, some good quality pellets, vegetables such as broccoli, cooked green beans, peas and corn, carrots, silverbeet, celery, oregano, spinach, rosemary, a cuttlebone for calcium and treats such as sunflower seeds, grainy bread, rice, sprouts, and millet spray. Most cockatiels aren't willing to eat fruit so don't worry if your bird doesn't touch it.
You can give him vegetables daily, and millet spray usually a couple of times a week or less. Millet spray is good food bribery for taming your cockatiel too, so if you only give him millet spray when you are training him to get used to you, it works well. Hope this helps.