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 fecal test will also look for coccidiosis and other more harmful worms. You can spend a lot of money on medications, but with the fecal test, you can know what you are treating.
I would get him taking some poultry NutriDrench now, consider the Corid in the morning, and get a fecal float on Monday. You could mention that you are concerned about E.coli and blackhead as well.
Liver disease might also color the white urates in the droppings yellow. More poop pictures are always welcome.