You need to take your bird to an avian vet as soon as possible. Crop burns are very serious and in most cases require surgery to remove the damaged sections of the crop. He will be left with a smaller than normal crop if the Vet can manage to save enough of the crop. There is no guarantee that this poor chick will even survive. He should have been seen by a vet as soon as the first burn happened. When a crop is burned, the damage can take several days to show up and by then infection has often set in. In this case where the burn causes a hole all the way through the crop, it is a very serious burn. It also leaves the crop sensitive where the formula has to be fed in small amounts and not as warm as usual. This is the reason why only a professional should hand feed a baby bird. Microwaving formula can cause hot spots if it is not stirred enough and even making it with hot water can result in formula that is too hot. The temperature needs to be tested in several areas of the formula to be sure it is all the same temperature. I am sorry to tell you but it may be too late for this little guy, however his only chance is to be seen by a veterinarian. In the future, please let the breeder wean the bird for you.
Thank you for asking Lafeber,