You don't mention where you are in the world - are you in the US or ?
I'm sorry that he's weak. Getting the fecal float would be good. Coccidiosis can be treated with Amprolium (Corid) if you are in the US. You can find it in feed stores like TSC. Coccidiosis is caused by a protozoa which is common and found in soil and in poop. Symptoms include poop with mucous or blood, lethargy, going off feed, fluffy up/unkempt appearance.
Parasites like worms can also affect chickens, you would need to use a de-wormer to treat those.
If you treat for Coccidiosis and use Amprolium (Corid) make sure that's the only thing in the drinking water. No other additives or extra vitamins until you finish treatment.
I would encourage him to eat his chick starter, you can make it into a wet mash if you wish. Sometimes bits of scrambled egg is also of interest to a sick bird. Hydration is most important, so keep getting fluids into him.