Could be aflatoxin in the feed, or some other fungus.
There is a sticky on how to do a flush, which is needed in extreme cases, you need to go with your instincts on that.
In the meantime and in any case, get some activated charcoal supplement - it comes in a powder or capsules. It is food grade, has nothing to do with charcoal briquettes for grilling. Drugstores carry it, some big grocery stores have it in their drugstore area, GNC-type stores probably have it. Call ahead. Put a tablespoon in a pint of water and get him to drink out of that. It is not harmful, it binds to toxins in the gut. It is used to help detox overdose patients sometimes.
I would put charcoal in everyone's water for a few days.
If you are in a bind and cannot find it to buy, if you know anyone who burns only clean hardwood, no scrap or treated lumber, you could grind charcoal from their wood stove really fine and that ought to be the same. But you must be absolutely sure of the source.