Your son has a systemic yeast infection due to all the rounds of anti-biotics he had to take because of the strep throat. The pro-biotics are helpful, as is the low sugar diet, but apparently it's not enough. I would go to a really good naturalpath or nutritionist who is familiar with chronic yeast infection. They will probably recommend no wheat in the diet for a while, no supplemental yeast of any kind. No white potatoes, no white rice. Garlic is a good anti-fungal. So is colloidal silver. I would think that he might be deficient in zinc, but that should only be given in low doses, especially for a child. I have to get it in liquid form to control the dose. Apparently you can also rub oregano oil into the bottoms of his feet (it's very strong and h*** o* the digestive tract for some people). For an adult, it's good to feed lots of salads and vegetables, and keep it to complex carbs, nuts, etc. for snacks. I know that's really h*** o* kids, though, because all kids foods and snacks seem to be high in simple carbs (like chips and crackers). Good luck!!
Good luck!