First, you must get water in him ASAP. Chickens must eat and drink all day long. If he hasn't had much to drink, help him get it. Add some electrolytes and/or sugar to the water if you can. That will help his energy level quickly. Food is the next thing we need to get in him.
When we have sick birds that won't eat, we make a mix of Starter Feed (Hatch to Hen feed is fine), Yogurt (any flavor; we used "mixed berry"), and vitamins (3 to 4 drops of Nutri-Drench is good here, but any Vit B-Complex will do, even tablet form if crushed and mixed into what we're making here). To sweeten the mix, add some fried or boiled egg yolk and mix all of this into a moist paste that he can easily swallow. If your mix is too wet, add some more feed; if too dry, add some more yogurt. You want it to be about the consistency of a moist dough ball like you'd make cookies with, not too wet, not to dry...
As for what's going on, this seems like intestinal parasites to me since the rest of the flock seems ok. Has he been wormed? Have you checked his poop to see if you can actually see worms in his stool? Also, check his vent to see if you see any external parasites. BOTH internal and external parasites will sap the strength of the even the healthiest bird. Many on this site treat with Safeguard for goats. Follow dosing instructions on the label as the dose is determined by body weight.
There is the possibility of respiratory infections and illness, but this really seems like worms to me. In either case, he is likely going to need to be hand fed and watered for the next few days. Are you able to do that? You'll need a few things to hand water, but hand feeding is as simple as holding him under one arm and gently opening his beak with your thumb and index finger of that same arm, then drop a small ball of the mix we created above into his mouth with the other hand. Hand watering is a bit more tricky, but the process is basically the same. Use a medicine dropper or needle-less syringe to place water into his mouth. Here are some good instructions for doing that as you must make sure you don't put water in his lungs or aspirate him. Read about his anatomy and how to water him here:
https://bitchinchickens.com/2020/02/06/how-to-safely-give-oral-medications/
If you do nothing else, you MUST get water in him ASAP. He will not survive without it...
S