Is he able to hold his head up? Hard to tell by looking at the top of him.
Keep trying to get fluids into him, if the crop was not empty this morning, I would not push a lot of food - see if fluids helps break whatever is in there down and moves it out.
His poop looks mucousy. Have you ever...
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Can you post some photos of him and his poop?
Can you see if he was pecked in the head too besides the tail? How long was he separated before the symptoms?
I would check to make sure his crop is emptying, look him over really well for lice/mites.
Huddling - he may be...