Most times a "too soft" stool is a product of something the dog is eating. If you try cutting out the treats, you can use his kibble as treats. Just give him one bit at a time for training and he'll think it's something special!
If his stool gets better, after cutting out the treats, then you know the cause and can adjust accordingly. If his stool is still soft, then you may want to try mixing in a better quality dog food. Not every dog can eat the (upper end) of the regular priced foods. Some dogs will forever be on more expensive food, as the ingredients are better quality and easily digested. He may have very a very sensitive digestive system and can't handle less than prime food.
I'm firm - but fair. Mind the rules and enjoy your stay.
According to the Humane Society of the United States:
There are an estimated 3-4 million dogs and cats euthanized each year in the US alone! PLEASE - spay and/or neuter your pets!