Oh that poor dog suffering like this for over two years. Melissa i would think that after two years she would have succumbed to stomach cancer already and the vet would have found cancer with all the tests.
Since the vets aren’t helping I’ll give my advice for what it’s worth. Just wondering if he’s on any flea tick meds or any meds at all that would cause constant stomach upset. Also have you tried for lets say for a week just boiled chicken and rice to see if the food is the cause? Make sure he eats some in morning and night as not to ever go with empty stomach. if the symptoms stop then you know if its the kibble or whatever food you feed . I know sounds simple but it’s worth a try.
