The met was most likely to reduce inflammation in the bowels though I've never heard of it used for poisoning cases, very odd.
But if you have black/tarry stools it's upper GI bleeding, is he throwing up any blood? I'd force feed an antacid (10 mg famotidine) for a few days along with some calcium carbonate (tums) to absorb some of the phosphates he's filtering through his kidneys, too high a level will damage them. And with poisoning, kidney shut down is all too common.
For food, get an oral syringe for horses, or as big of one as you can find and pack it with something bland to keep from further damaging his digestive system. White rice and ground beef is good, just puree it and add enough broth to make it easier to force feed. Force the gatorade, as much and as often as you can!!
Don't know why the vet would send him home, he needs to be on IV fluids for a at least few days or at least have sub-q available at home.
Hope that's helped a little.