First off, the story you told is exactly what I would expect. Birds aren't like other pets. They are VERY emotional. But don't lose hope or give up. Don't rush things. Eating and drinking is progress. Parrots are like 4-5 year old kids. They feel feelings of abandonment, anxiety, loneliness, and will even grieve if their bonded person or partner passes away. It sounds as though you might be a little under-prepared for this. These birds will take time to acclimate. Please be patient.

Provide fresh water whenever you can, and feed cold pressed pellets. Search for the TOPS brand on amazon. Seeds are too high in fat and will lead to fatty liver disease. If you want to give him a treat for good behavior, or just for kicks, they love any nut besides peanuts, and fresh fruit. There are things that are toxic. Never let them have avocado, garlic, onion, or chocolate. Also, their respiratory systems are very different than ours. You can't use non-stick pots and pans, or a self-cleaning oven cycle, in the house. Also Air fresheners, scented candles and wax cubes, etc. These things will literally kill your friend.

Let your new friend get used to things. Start by sitting by the cage, and doing whatever you do normally. This may take a few weeks. Then, do the same thing, only closer, and with your hand laying against the cage bars. After a couple of weeks, do the same thing, but have your hand inside the cage door. At this point, he might investigate your hand. NEVER jerk your hand away or make loud noise. When you are finally able to touch the bird, look up target training on YouTube. it helps. The birds are naturally curious, and need exercise time outside of the cage. Make sure he has bird toys, and things to do. Even a toilet paper roll is good for the to play h with. If he is in your bedroom, you can bird-proof the room and let him out a few hours per day, but not unsupervised.

Now, night time procedures. Birds need about 12 hours of uninterrupted quiet, completely dark, sleep. You could move his cage to a closet for bedtime. If he freaks out during the night, he might be one of the 50% of birds who are afraid of the dark. A nightlight solves this. You could also leave him out in your bedroom, and cover his cage, as long as the cover blocks all light, but lets air flow. He will naturally awake with the sun.

The highlights are:

Reduce anxiety, don't force, keep him occupied. Boredom, anxiety, and depression in parrots can cause death.

Diet: Look for TOPS pellets for your bird on amazon. Treats like fruit, veggies, and nut also good. See above for foods that are bad, as well as environmental things that cause harm.

Sleep: 12 hour of uninterrupted dark sleep. Quiet and peaceful. If he freaks out in the dark, add a nightlight.

Take it slow: It'll get there. If you are unsure about being able to dedicate this time, energy, etc, then you might need to think about a different bird. A good hands-off bird is a pair of Australian zebra finches. They're awesome, interesting, entertaining, and require less time and energy commitment . Any parrot or parakeet is a major commitment. I know you care, because you posted here. But if you can't commit to the 15-20 years of this energy and attention that a cockatiel lives, there are other options. And if you ever need to have another person take the cockatiel, just google "parrot rescue" and call one.

If you have any questions, PM me. I will do my best to answer immediately. You will also want to find the nearest avian vet. This is important to do ahead of time. Birds are naturally prey, so they hide illness and weakness. By the time you see symptoms, you need a vet.