I would isolate your rooster from your hens. Continue putting some vitamins/electrolytes in his water, and encourage him to eat as much as possible. Some good foods to get him to eat are moistened feed, scrambled or hard boiled eggs, mealworms, wet cat food, and bread. He needs to start gaining weight.
I would also recommend worming him with a broad-spectrum wormer. Some choices include Valbazen, SafeGuard, and the Worminator (Flubenvet). Don't use Wazine or Ivermectin, as Wazine only works on roundworms, and many worms are resistant to Ivermectin. Most wormers can usually be found at a livestock supply store, and the Worminator can be ordered online from here:
http://www.twincitypoultrysupplies.com/store/index.php?main_page=product_info&products_id=584
You might also consider treating him for Coccidiosis. Get some Corid 9.6% liquid or 20% powder; the dosage is 2 teaspoons/gal. for the liquid and 3/4 teaspoon/gal. for the powder. Treat for 5-7 days, and do not put vitamins in the water during treatment, as they will interfere.