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 hard on 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 hard on kids, though, because all kids foods and snacks seem to be high in simple carbs (like chips and crackers). Good luck!!
Good luck!