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