Allergies and reoccuring yeast can by symptoms of a bacterial overgrowth which just adding a probiotic alone will not correct. There is definitely something deficient for this to be happening. Sometimes, being a "picky" eater and not eating for days can be a symptom that his belly isn't feeling well.
All that being said, we use a daily probiotic. I love the product by askariel.com. Although it isn't cheap, it also isn't cost prohibitive. I give it to both my girls, and you only use 1/2 capsule once a day. You might want to give Susan Davis' site a look around. She has many supplements that help with allergies and yeast overgrowth. Perhaps a consult may be indicated to address his nutritional needs/picky eater as well......
Pet Health Pet Nutritionist Holistic Pet