Baby chicks that are not thriving will stay foo by themselves. Try to give it a lttle more attention: is it drinking enough or eating enough? You might temp the appetite with some good quality yorgurt, like Stonyfield. Plain. Mix a small spoonful with crumbles. Without attention, little ones go quickly.
I would do the ACV any way--easy cleaning and good health promoter anyway. Perhaps a litlle sugar in the water--go look at the hatchery websites for amounts, about 1 tsp per gal. Give electrolytes if you have it. Google gatorade and see if that is used with chickens as i use it for lambs (lemonlime) Feed stores often have small packets of electroyte/mineral/vitamin concentrates to add to water.
Maybe a little soon for coccidia, if it hasn't been out doors yet. GOOD LUCK.