Welcome to BYC, sorry your poult is sick. If it were mine I would treat it for blackhead, E.coli, coccidiosis & worms. Is a vet an option?
Metronidazole for blackhead - 23 mg per pound orally once a day for five days.
Baytril (enrofloxacin) for E.coli - 7 mg per pound orally twice a day for 3-5 days.
Baycox (toltrazuril) for coccidiosis - 10 mg per pound orally once a day for 2-3 days.
Safeguard (fenbendazole) for cecal worms - 23 mg per pound orally once. Repeat in 10 days.
I would also tube-feed it fluids. Once hydrated, I would tube feed it baby bird food.
Tubing
Step 1:
Bring poult inside and place in a warm room, 80-85 degrees is ideal (watch for signs of overheating).
Step 2:
Weigh the poult
Step 3:
Once warmed, correct hydration, and this should not be done until the poult is warmed up. Tube warmed (102 degrees) Pedialyte or Gatorade at 14 ml per pound of body weight, wait 60-90 minutes and repeat. If no poop is produced by 3 hours after the first tubing, repeat once more.
Step 4:
Once the poult is pooping, you can start tubing warmed Kaytee Exact baby bird food or a non-lay crumble (lay crumble has too much calcium). Start by tubing 14 ml per pound of body weight and increase a little at each feeding. Do not exceed 23 ml per pound of body weight. Sick birds are tube fed 2-4 times a day.
"Tube feeding, also known as gavage feeding, is an essential part of avian supportive care. Sick birds are often presented with a history of anorexia, and glycogen stores may be depleted within hours in the granivore (including psitacine, passerine and galliform species) secondary to a relatively high metabolic rate. Another important indication for gavage feeding is a documented drop in body weight of 5% to 10%."
Source:
https://lafeber.com/vet/tube-feeding-birds/