Unfortunately, if you have Mareks in your flock, that is a good possibility. Since there was blood in the dropping, I would get some Corid to put in his water for 5-7 days. But if he won’t take enough water, you will want to give him a concentrated oral drench daily. Later, after a few more days, I would think about worming him with Valbazen 3/4 tsp once and agian in 10 days. I would keep him propped up or in a chicken sling, so that food and water can be placed right in front of him. Slings are good to get them up, but they need to get out of them to stretch and sleep every few hours. If you lose him, I would save his refrigerated body, contact your state vet to get a necropsy to look for a diagnosis. Here is a good link for state vets:
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf
https://www.aphis.usda.gov/animal_health/nahln/downloads/all_nahln_lab_list.pdf