With sick chickens I offer wet chicken feed with a lot of wtare, some plain yogurt (about 5 ml,) and offer chopped egg or tuna to get them to eat. You may also learn to tube feed him with a feeding tube and large syringe from your vet. Google “Go team tube feeding” or look at some crop feeding...
Where are you located? Newcastles has 3 types, and the exotic type is not in the US. Did you get a diagnosis from a vet, or what symptoms were you seeing that led to your diagnosis? Exotic Newcastles is such a serious disease here in the US, that birds would be culled and incinerated.
More than...