Did he have a food change to cause the diarrhea? I have found that the dog probiotics that you can buy in the syringe is the best. I can buy it in our local store, but I don't think you can get it everywhere. I first got it from my vet. If he hasn't had a vet check and he is new to you, I would take him to the vet, you don't want your other dogs infected with anything he could bring in. A stool and blood test could be very worthwhile. He isn't going to come around if he is ill.
Started Primal June 2012 at 148.5lbs, and 5' 1", reached goal weight in 5 months.
Lowest weight 93lbs - too thin. Now stable at 102lbs much better weight for me at my age.
Primal, minus eggs, dairy and a myriad of other allergens.