He may be dehydrated or hypoglycemic.
Put a drop of pancake syrup in his mouth to raise his blood sugar.
Then begin by giving him two droppers (cc size) of unflavored pedialyte every ten minutes for an hour. Next water down(plain water) canned cat food or baby food chicken or turkey with no onion or garlic and give only one dropper every ten minutes for another hour. Wait two hours and repeat both. When he feels hydrated he will lick up the gruel on his own.
Make a temp electrolytes solution by adding an eighth of a tsp each of salt, sugar and pancake syrup to 12 oz of water.You can use tuna water or milk to hydrate as well
Boil chicken or fish as the scent of food cooking awakens the appetite sensor in the brain.
Please get started ASAP and let me know how things are going. I will stand by to hear from you and help further.