So this is a very young baby that you are hand-feeding formula? How old is this chick?
Does this chick have all of it's down feathers in yet? How about it's outer feathers?
Are you keeping him in a Brooder with a controlled ambient temperature at all times? Are you making sure that all of the formula you are feeding him is between 104-110 degrees F by using a cooking/candy thermometer?
Is his crop almost emptying between feedings and then completely emptying overnight? Can you check the outside of his crop, on his chest (meaning the outer skin covering his crop) to see if it appears at all red or inflamed?
There are a lot of possibilities here, but the most common are that the bird had some type of fall, or was dropped, etc. and has some type of internal bleeding; The formula you have been feeding the baby was hotter than 110 degrees F and has caused a crop-burn; or the baby has some type of GI parasite or infection. I need more information about your set-up and your processes to be able to help you further...