Why does my baby lovebird not eating?

I think you should try to see the vet earlier. Baby birds need adequate nutrition in order to survive, and their health can decline very quickly. There may be a number of reasons why he is rejecting his formula. It may not be the right temperature or consistency for him. Or, he could be feeling ill. Baby birds can get an infection called "sour crop." This infection causes their crop to slow down, digestion slows down, and they lose their appetite. You also could have burned his crop if the formula was too warm. This would also cause him to lose his appetite. So, if you love your bird you should see a vet right away. His life may depend on it. Good luck to you and I hope that he feels better.