I'm going to post this here so you can print it for reference in the future ok?
There are quite a few possible causes for what you're seeing and in more than 90% of all presentations it's usually nutritionally related. So let me go over that aspect and the other top possibilities:
Nutritional problems are really the number 1 reason for feather loss or plucking in the vast majority of cases. Sometimes nutritional problems lend to liver problems and that by itself is a cause of feather plucking. More on this further down
The next most often issue, if there’s a cage mate (I know you said there is not, but if it ever happens), is over preening by that mate or aggression. Sometimes it's both. A cage mate might be frustrated, feeling stressed for any one of a hundred reasons birds have and they'll take it out on the nearest subject - which happens to be the other bird.
I've seen plenty of birds like this 'preened' until nearly bald! What's worse is that they're having blood feathers (newly grown) pulled out and that's got to hurt.
You don't mention the size of the cage, but if it's not at least 2' x 2 x 3' you may want to consider upgrading their house. Remember, his natural home is the wide open world.
So a bigger cage (and a nutritional modification ) might resolve everything
Nutritional problems are really the number 1 reason for feather issues in the vast majority of cases. Sometimes nutritional problems lend to liver problems and that by itself is a cause of feather plucking.
A good base of pellets or crumbles specifically for 'tiels is available from various sources. One popular brand is Harrison’s, but as long as your bird is eating any kind is better than eating none.
Supplementing this diet with fresh foods every day is ideal and many owners find they can re-introduce seeds - in limited amounts (perhaps once or twice a week) without the bird refusing the pellets overall. To be honest, the more research that’s done, the more it’s shown that seeds shouldn’t be fed at all anymore. Remember, we don’t own wild, foraging birds that are spending 12 hours a day in motion, flying for miles and miles dawn to dusk, doing what birds do.
Whole grains, dark leafy vegetables, fruits and legumes. Include the colors orange, yellow , green, plus reds too! Think sweet potatoes/yams, squash, melons, oranges, peas, chard, beets and others.
Brown rice, quinoa, whole wheat couscous and natural, whole grain pastas are great choices.
Limit fats, especially the kind from animals. Good fats are most plant fats like soy, olive and canola oils. No fried anything
Another thing you can try is all natural, human baby food. Stick to the orange colors.
They can be mixed with tiny pasta or rice, whole grain bread or toast - remember, be more creative than the bird is stubborn.
Now, with all of this said and suggested, it's time to have a vet check up for a few reasons:
First to make sure there isn't a disease behind what you're seeing. A sample of droppings, an oral swab and perhaps a swab of the raw area under the wing will be done. I'd also like to see a blood test to be sure there isn't a heavy metal toxicity (from toys or even the cage bars) or other issue happening.
Second, to establish the bird and a baseline so if anything arises in the future, absolutely anything, you've got an 'in' when an emergency happens. Many vets will not see a new bird as an urgent care patient. So if you've been to the vet even once as a regular patient, you get seen when it's really necessary.
I highly recommend at least once a year check ups. It's generally less than $50 which equals like 13 cents a day. After all, these birds are like our children right? They depend on us for everything and they love us unconditionally.
Until you have him seen, you can lightly dust corn starch under his wings on any raw areas. If a feather is bleeding and won't stop, smear the end of that feather with corn starch and it should stop the bleeding within a minute. Use all the corn starch you need for this and try to keep him calm. A frightened cockatiel tends to flail around
So finally, if some of the hard flights in his cage are seemingly out of the blue and at night you can be pretty sure he's seeing a shadow, hearing a sound or something else that triggers the 'flight or fight' reaction. Cockatiels aren't much for fighting
If you cover the cage for bedtime, be sure to leave a portion uncovered so he can see out and feel reassured he's safe
During the day be sure he's not seeing something scary for a bird on tv or out a window. One of my clients thought she was doing her bird a favor by leaving a nature station on for him. Filled with images of predators like raptors, snakes and cats pouncing on various prey only left her poor bird a nervous wreck! When she left cartoons on for him (pre school cartoons) within days she noticed a big difference
I believe you're doing a very good job and I'll work with you however you need to make this all better for you AND your bird