Butter oil is the clarified butter fat portion from cream, milk or butter and contains 99.7% milk fat with not more than 0.05% nonfat milk solids (same as skim milk powder). Butter oil is the same as ghee used in Indian cooking.
These chocolate chips are not lactose-free because milk sugar would be found in the nonfat milk solids the same as in powdered milk that you can buy in the store. While butter oil is almost all butterfat, it still includes a trace amount of nonfat milk solids. It is not free enough of lactose to be allowed on your son’s lactose-free diet. I would not suggest using this chocolate chps.
You could use small chopped or grated chunks of dark chocolate without butter oil in place of the chocolate chips in the muffins. Make sure to read the ingredient label of a dark chocolate bar, not milk chocolate, that it only contains chocolate and sugar.