I’m very sorry to hear about this little chick. At this point, it needs to be treated by an avian vet as soon as possible. It sounds like the chick was not eating well on his own and should not have been sold. At 45 days, a chick is too young to be fully self feeding. A cockatiel chick should be supplemented with hand feeding until between 8-12 weeks old. In the wild, the parents would actually help it for a few more weeks while it learns to find food. While you did the right thing by handfeeding it, what you were feeding had little to no nutritional value. The chick needed a proper handfeeding formula. You were keeping the little bird hydrated, but he was basically starving. He most likely stopped eating because he had no more energy. It’s never a good idea to force feed a bird. If a bird will not eat willingly, it needs to be taken to a Vet immediately. The Vet can safely feed the bird with a tube. But more importantly, the Vet can determine why the bird isn’t eating. A bird has to be very sick or weak to stop eating. When they reach that point, they will not survive without Vet care. When you force fed it, the bird probably aspirated – inhaled food into his lungs. This usually happens when you try to force feed a bird. At this point, he needs emergency Vet care. With immediate care and a skilled Vet he might can be saved. Without it, he is most likely going to die soon. I’m sorry to tell you that, and I’m sorry someone sold you a bird that was too young without showing you how to hand feed it and giving you the right kind of formula. I would not get another bird from the same place.