What signs has your cockerel been showing that looks like coccidiosis? Symptoms can include lethargy, weakness, standing around puffed up or hunched, not eating, and runny stools. Corid is meant to be put into the water, and they will drink the proper dosage for their weight. You can give a few drops of undiluted Corid a couple of times a day to jump start or increase what they are drinking. Using the 2 tsp per gallon of water disage for the liquid is good for 5-7 days. Give 1.5 tsp of the powder per gallon if you are using that.
Could he just be maturing and acting more aloof and different from the pullets? You can get some droppings checked for coccidiosis and worms, if you take some combined ones into a local vet, but always call ahead to make sure they will help.