I probably would have continued the Corid in case he has coccidiosis. It is not harmful or an antibiotic, and should be given for 5 days to 7 days. If you are not going to give it, then I would give him the Poultry NutriDrench vitamins daily. A small spoonful of plain yogurt mixed in his feed...
Chickens usually do not suffer from blackhead. It is more of a turkey disease. I would call a vet early Monday, and take in some fresh droppings for the fecal float. They will look for worms, including cecal worms that are a cause of blackhead, but which chickens are not so bothered by. The...
Can you post a picture of his droppings? He could possibly have coccidiosis with his runny poops. Bright yellow poops like mustard can be common in E.coli infection.
I would get some Corid (amprollium) from your feed store and start treatment for coccidiosis. Until then, if you have some...