I would treat for coccidiosis and worm him. Separate him which if you have others they may already be exposed. But he sounds like he needs extra care. Corid is one of the best coccidiosis meds and I would worm him with panacur (safeguard) 10% suspension for goats. .5 mL (1/2 mL) orally down throat for 3 days. Then repeat same dose in 10 days. Are they on the ground outdoors? With other poultry?
I would also keep him warm, try to get him to eat, mashed up hard boiled eggs with some very small pieces of bread, anything soft that has high calories and protein. High protein isn't recommended for healthy chicks for extended times, but if he is lethargic like you described he needs it now.