I wouldn't use yoghurt myself, dairy is known to cause diarrhoea, so unless you know he tolerates it well, I wouldn't risk it. The good stuff found in the yoghurt is available from health food shops, ask for probiotics.
How many meals are you feeding? Like Velma says, several tiny meals are the key. If you can't get canned pumpkin, use fresh and boil/mash it. Butternut squash is an easily available alternative, any fibrous veg (not green/leafy)in a tiny quantity will work. I think vets on monday if he isn't fully recovered.
When was he last wormed? Worms and worming medication can both cause diarrhoea. Just a thought.