First, get him inside and warm. AND AWAY FROM THE OTHERS, just in case it is Merricks.
Sugar water for a quick energy boost.
Then a mixed paste of nutridrench, a small amount of coconut oil and St. John's wort once a day. (Just a pinch of the St. John's wort.)
Poultry booster pellets from rooster booster was extremely helpful to add when my girls started eating again.
Vetrx had to be purchased when they had some upper respiratory symptoms.
One girl I had to tube feed for a week.
Finally, I gave both mine penacillin injections for 14 days.
It's very time intensive, less than scientific, I kinda threw all I had at them, and if you can not put in all the effort I would give the vitamins a try with st. John's wort and let him sit while you pray and hopefully he would perk up!
I also had to reteach one to walk again by making a chicken sling since her muscles wasted.
Keep trying to get him to move his legs and wings so they don't stiffen and lose condition.
