If the baby is being fed formula, I see no reason why a little flax oil could not be added. Formula contains added fats which are not the same as breastmilk so it is not like the baby has not already been introduced to something different. Confirm this with your pediatrician but I would be leary if he wanted to prescribe drugs for constipation.
My old Sicilian mother in law says they used to use a parsley stem with olive oil on it to stimulate a BM.