HELP! Horse really sick.... chronic diarrhea!!!

First step, find a new vet. Have the new vet run blood work, culture the stool, and do a full exam.

If he were mine, I'd put him on 3-5 lbs of hay pellets (alfalfa or alfalfa mixed) per feeding, 1 cup stabilized rice bran a day, GUT supplement (has probiotics in it) or Probios, and Aloe Vera juice (you can buy it at a health food store). Give 5x the dose of Aloe that is recommended for adults.

Spike his water with electrolytes and some molasses, to encourage him to drink more. Give him lots of turn out time, in a pasture or paddock by himself. There are 1,000,000x more germs and pathogens in a stall than out in a field.

Good articles on Diarrhea:
Treatment for Equine Diarrhoea
Horse health: Diarrhea causes and treatments - by Maureen Bordelon - Helium

Find a new vet.

The internet is a great way to network and gather information but if this is as serious as you say, you need a real live in person doctor.

I agree, but make sure you have one bucket of just plain water. Don't put electrolytes in all of it, just two buckets and then a third with pure water. But I agree, call a new vet ASAP.

Couldn't agree more. Instead of asking for fast help here that could endanger the horse further, call a VET!!!

I don't know if anyone else has said this, I don't want to read everyone elses posts, but one of the horses at my farm is older, and he had veryyy bad diarrhea, our vet said to put a table spoon of pumkin filler in his feed once a day. You can find it in a can at your local grocery store. Its the filler you put in a pumkin pie. Its helped him SOO much. He's gained weight, everything!