I don't think it is cocci because they don't have bloody stools. For a large shot of energy you could try Nutri-Drench, but that will just give you more time to find out the problem. Just a thought, I found this on the Internet, see if it's a possibility...
When you notice a young chicken lame in one or both legs, perhaps his wings dropped down unnaturally, mouth gaping trying to breathe, it is time to consider the possibility of an active Marek's disease infection.
Marek's has four typical expressions: ocular (eye), cutaneous (skin), neural (nerves) and visceral (internal organs).
In the ocular form, the shape of the chicken's pupil deforms (it should be round) and the brown iris turns gray. Light sensitivity occurs followed by blindness.
The cutaneous form displays reddened, swollen lumps at the site of the feather follicle.
In the neural form the sciatic nerve is swollen causing lameness to the point of paralysis in the legs, dropped wings, “gaping” (gasping for breath), weight loss and diarrhea. The birds can't get to food and water without assistance and may be trampled by other chickens.
The visceral form is only observed by an overall wasting. During autopsy tumors on many of the internal organs can be seen.
Occasionally a chicken will seem to resolve his symptoms and function normally for a short time. This is almost always followed by death due to the growth of internal tumors.
