I would go ahead and take him to a Vet. Handfeeding birds can get an overgrowth of bacteria or yeast sometimes. Something is bothering him either in his throat or crop. It’s also possible that you are overfeeding him or feeding the food too hot or cold or too thick. When you call the Vet, you need to schedule an appointment at one of his feeding times, so that his crop is almost empty when you take him. Take some of the formula with him in case he needs to be fed some after the exam. If he has too much food in the crop, the Vet can’t examine him without risk of aspiration. If you do feed him before you leave the Vet, just give him a small amount in case he gets car sick. If he does have a yeast infection, the medication generally needs to make contact to work, so it would have to be given to him about 15 minutes before you feed him to give it some time to coat the crop. If you are using a heating pad to keep him warm, take that with you, too, so he doesn’t get chilled. He isn’t old enough to regulate his body temperature. I’m sure he will be fine, but don’t wait about taking him or he could get too sick to help.