If you changed your pup's food cold turkey, it can really mess up their digestive system. Food should be changed gradually over a 2-4 week period. You don't want to cause a bout of pancreatitis. Also, a large breed puppy shouldn't really be fed a kibble with protein higher than 28%. Too high of a protein can make them grow too fast, causing joint problems down the road.
The mushrooms concern me too, since many are poisonous. A trip to the vet may be in order. I wouldn't leave the pup unattended in the back yard at this stage, b/c they WILL eat anything they can get their mouths on, as you've already experienced.
Has your pup been tested for parasites recently? Worms can cause diahrrea.
You can try adding a spoonful of plain pumpkin (NOT pumpkin pie filling) to each feeding to help firm things up. I do think you should schedule an appt. with your vet.