I fed TD Kaytee's Exact baby bird formula. I used a syringe. You have to press it against the side of their beak (at the back of the beak to get them to open up. It only took 2ce for TD to just start doing the dinosaur screech and the begging whenever he saw the syringe coming. Do you have a digital scale? Idk how much a bird that age should weigh but you'll be able to see if he/she is loosing weight and if so you may want to try the formula for a few days. In this picture, bottom right, you can see the tip of the syringe I used. He went from that to a spoon quite quickly.
One way to encourage them to eat seeds is to put the seeds by them and tap your fingernail on the table, to imitate the sound of a bird pecking seeds. This worked wonders for the two wild fledglings I raised. TD also will begin eating if I do it
Idk about other cockatiel owners, but, all three of my birds eat better if I eat with them. They do this thing, I call it the eating game or the food race. You have to play it with them. It sounds silly, but you keep your eyes down on your food, and every so often glance up (raising your head) and look at the bird. As soon as he looks back at you drop your head and continue eating, then do it again. That seems to be a "go" signal. You have to do it throughout the meal. You'll see them doing it to you as well. I thought this was just a little game between TD and I, but then I got AthenaDon and I see them doing it with each other. Even Oscar (the Quaker) does it, though not in such a rushed, racing way.
Good luck. Keep us posted.