You can put a lot of water in his chicken feed to get fluids into him. Add water throughout the day to keep it moist, but change it out each day. I kept a hen alive with water in her feed for 5 weeks until she was able to get up and walk after a long illness, and she did not drink any water. Green droppings are a signal that he is not eating enough.
If Phil has a vitamin deficiency, vitamin B2 riboflavin is the important one to give, and B complex has that, but B 12 is not the one you need. You can buy B complex injectable at most feed stores in the cattle dept.
https://www.tractorsupply.com/tsc/p...mplex-fortified-injection-100-ml?cm_vc=IOPDP1
As long as his crop is emptying normally, he should be absorbing the oral vitamins.
If he has Mareks, he may not get his legs back working. As long as they are eating normally, they can live until tumors or infections become a problem. It is best to get a necropsy by your state vet after you lose one, just to make sure what disease they had. Local vets are not usually able to diagnose Mareks.