It sounds very much like your chick's crop has started to slow down. A trip to the vet whilst the crop is up is advisable as chances are he needs some antibiotics. The causes of sour crop or crop stasis as it is known are the food is too cold (needs to be between 38 and 42 degrees celsius, anything lower will cause the crop to slow down and anything higher will cause burning and possible death), the chick's environment is too cold or not constant and the humidity is too low.

It is very, very important not to top us feed until the crop is completely empty. All you do then is cause the food to sour in the baby's crop making the problem worse. A lot of people swear by giving a little apple juice when the crop is up as this is said to neutralise the crop. Your vet may want to flush out the old fermented food out of the crop and administer medication directly in the crop. Sadly it appears that once a chick starts with crop stasis chances are it will be an ongoing problem. If left untreated it is fatal.

Try reading this as I found it very helpful Winged Wisdom Pet Bird Magazine - Sour Crop in Cockatiel Chicks

Keep baby warm and seek veterinary advice. Good luck x