Continually sloppy poos can lead to potassium loss, if not treated this can be serious
http://en.wikipedia.org/wiki/HypokalemiaAlthough tablets are available for this, they are not the route to take - they can cause severe gastric pain and other problems (as I know to my cost)
I'm wondering if the dog is intolerant to wheat or some grains? Intolerance to one type of grain doesn't necessarily mean intolerant to all. eliminating all but one single grain from the diet may help to discover this but can take months to do, working systematically through the different grains.
As an alternative, what about raw fed but with the addition of potato or rice - if tolerated? Or maybe a food based on potato or rice with fish instead of meat if they haven;t already been tried?
Remember, where the gut is sensitised or damaged it can take 6 weeks, or more, for this to heal and show improvement in the 'produce' at the other end ;)
In the short term, green bananas (ones with green on the skin) can slow things down, as can cooked sweet potato or pumpkin. Grated Courgette are also good.