Myles was getting 30 mg a day at 17 lbs, so the dose sounds just right to me.
As for the lactose, if your son is MSPI that means he’s intolerant to the proteins in milk and soy, not the sugars. Lactose is a milk sugar. He could be intolerant to that too, but it’s much, much less common for infantst to be lactose intolerant, as mother’s milk also contains lactose.