Isolate him from your other chickens. Keep him in a warm dark room.
Check over his entire body for injuries, bites, and swelling.
There are three other things you can do that will not cause him any harm, even if they do not help.
1. You can keep him hydrated with a 50/50 mix of Pedialyte and water. Put drops on his beak and see if he will suck it in. You can use a syringe to get water down his throat if you can prop him up and make sure you don't get the fluid down the airhole under his tongue. Put the syringe back far down his throat.
2. You can get some liquid baby vitamins with no iron (Poly-Vi-Sol) and put three drops a day on his beak for a week.
3. You can give him aspirin if you think he is in pain: 1 baby aspirin in 1 cup water or up to 5 325 mg aspirins in a gallon of water.
For any further help, you need to tell us his age, what you feed him, what his poop looks like, if his crop is full or empty.
Good luck.