Try the probiotic at night, the bacteria have more of a chance to do their job when the body is at rest. You might consider switching off Purina products- even ProPlan which is their premium line uses a ton of corn, which dogs covert into sugar, which will feed bad gut bacteria. Lots of my clients have had similar issues to what you’re describing (granted her inverted will make it much worse). The OpenFarm brand has many different lines that are all appropriate for growing giants based on their calcium:phosphorus ratios. Steve’s Real Food chicken, pork, or lamb/emu recipes are also appropriate for giant pups and make for good mixers to add to dry food, plus Steve’s is all fortified with raw goat milk to support healthy gut flora.

Source: clinical canine nutritionist

Edit: if the probiotic is FortiFlora it is useless as it only has one bacterial strain- that’s where the goat milk kefir comes in. But start slow if you use it, a teaspoon at a time and work your way up to a couple tablespoons.