I think your son got abdominal symptoms due to some other ailment. You thought its cos of invisalign
as thats the only treatment he was undergoing. Its human nature to co-relate to extremely different things which co-existed. No medicine, no material, which is used / recommended by any doctor comes in the market without knowing its all positive and negative effects. Get complete medical examination of your son done by a good physician i.e MD (medicine) and get all the blood tests and sonographies as told by him. Remember one thing, if its due to allergy
, its symptoms will first show up locally i.e in oral cavity, lips, cheeks etc. I wish a rapid recovery of your son.