I would think a bland diet, but you should really discuss this with your vet, or a vet nutritionist. All manufactured dog foods already contain vitamins, so one needs to be careful when adding to the formulation. I would also seek a vet's opinion on the probiotics, as I am unfamiliar with their use in a dog with an ulcer. I wouldn't think they would hurt anything, but I'd feel better hearing that from a professional. Some contain digestive enzymes, so that would need to be taken into consideration also.
Hope little Dudley is feeling all better soon...
