Lalobear- My son and I are about 125lbs. We both take an adult sized dose. I would just cut the adult dose in half for your son.
His reaction will depend on how parasitized he is. If he has very few parasites he won't have much of a reaction. The unpleasant side effects occur when they die and the body has to deal with all of the toxins.
This may help your whole family! It sure has helped mine.