Plain boiled chicken & rice would be best w/added insoluble fiber like slightly cooked carrots or frozen green beans. Eliminate all treats during this time. Bentley is 9 months old, so you should transition him very slowly (14 days) to a new premium adult food, grain free preferably. Make sure the 1st 5 ingredients contain a meat source & is higher up on the list, as that makes up the bulk of the food. Some dogs are allergic to some grains like wheat, corn, or soy...it will cause loose stool if they're allergic to a certain food.
Best Grain Free Dog Foods
Has he been tested for parasites?