What exactly is his stomach problem? has a vet done any tests to tell you exactly what the problem is?
If I know my dog has some issues with foods, I wouldn't just give him anything a trainer recommends, while they may be knowledgeable in some areas they are certainly not nutritionists, heck, even vets sometimes are not very knowledgeable in this area.
Dogs don't need to talk, their life is their message ~ unknown.
"The greatness of a nation and its moral progress can be judged by the way its animals are treated." ~ Mahatma Gandhi