Glad the others have recovered. If he were mine I would try treatment with one of the other medications. It's possible that he had more damage to the gut from the coccidia also.
If he's still having droppings like that after 5 full days then you may have to treat with another medication. Occasionally someone gets a strain that is resistant to amprolium. Did any of your other birds show symptoms? If he's not showing any improvement then you may need to use a sulfa...
Any chance he's been eating berries or anything else that might color his droppings?
I would consider his treatment started on the day you KNOW that he only had access to medicated water, before that he may not have taken enough in. To clarify, correct dosing is 1 1/2 tsp of the powder or 2...