My kids drink milk (raw and organic) at all meals, water in between. I never buy soft drinks and we do not eat out of boxes or cans. We have some type of salad greens at least once a day. Homemade dessert is limited to once per week with Sunday dinner.
Still, my oldest is allergic to just about every household and outdoor allergen there is. His allergies lead to asthma and he has twice landed in the ER because none of his meds could get his breathing back to normal.
I do not know why he suffers so badly from allergies and asthma. He also exhibits ADD like behavior, we homeschool so it won't ever be diagnosed. Let me just say that I have a new appreciation for teachers who want kids on meds to help them focus. The stress that his lack of focus adds to our day is unbelievable. It is beyond the normal absent mindedness of boys his age. Why just him out of 5 children?
Odds favor that the asthma is secondary to allergies. Have you taken him to an allergist? If he gets his skin tested, maybe he can become desensitized with injections of the offending antigens.