You are grasping at straws. Your description of symptoms matches wet pox, but I haven't heard any that match infectious bronchitis or sour crop. Without seeing him, they cannot be completely ruled out, but the symptoms you describe sound like wet pox.
For that use qtips and listerine (the original very nasty tasting stuff) to thoroughly swab out his throat at least twice per day; 3 or 4 times a day is better. You want to remove as much of the white membrane as you can, but it will be painful and probably bleed, You are not likely to get it all the first or even second time. But you need to remove it as that membrane is making breathing, eating and drinking difficult. You also want to add an antibiotic to his water to ward off secondary bacterial infections (pox is a virus, so antibiotics will not help)
For that use qtips and listerine (the original very nasty tasting stuff) to thoroughly swab out his throat at least twice per day; 3 or 4 times a day is better. You want to remove as much of the white membrane as you can, but it will be painful and probably bleed, You are not likely to get it all the first or even second time. But you need to remove it as that membrane is making breathing, eating and drinking difficult. You also want to add an antibiotic to his water to ward off secondary bacterial infections (pox is a virus, so antibiotics will not help)