I would stick to one food, if he doesn't eat it, take it away until the next meal time. Dogs aren't stupid and they won't starve themselves, if he's hungry he'll eat........eventually.

I definatly wouldn't mess about adding more foods into the equation like eggs or yoghurt that will more than likely just exagerate the problem, when ours get soft/sloppy stools we don't change anything, just keep feeding as normal and it soon sorts itself out.

Having said that Chappie is not a good food and it is "easy to digest" so maybe that is why he's had soft stools; his digestive system is used to having to work harder than it would with the Chappie, just a theory but it makes sense to me!

If you want to feed yoghurt in future I would advise only giving natural probiotic/live yoghurts.