My dog had similar troubles a few weeks ago and nothing would shift it...she had it for over a month
She's fine now and eating like a horse, slowly getting back to her old self. I started her off on boiled chicken breast (no fat, skin or salt) and plain boiled rice, but the rice was coming out undigested, and the runs continued.
I read online that rice tends to ferment in their stomachs so took her off boiled rice and replaced it with plain boiled potatoes, still in skins (but washed first!)
mash em up with a fork in his bowl, mix some of the chicken/potato water with it to give him extra liquids as the chicken will dry it up again, into this I put one teaspoon of porridge oats, my dog has had solid dumps since then and I've slowly cut the oats down to a spoonful every few days.
The starch in the spuds tends to take the toxins from his intestines, giving them chance to heal and the oats slows down digestion, giving them chance to gain their strength from the food.
Don't overdo it with the porridge oats, you may wanna make his food half an hour before feeding time and add extra liquid if the oats turn it into concrete as they do absorb a lot of liquid.
This is also good for dogs that are allergic to wheat, which can also cause gastointestinal problems