Very simply, open carry is, and always has been, legal in the State of Alabama without a permit. When open carrying, the only time you must have a permit is when you get into a car.
A good place to start for the information you seek is http://alabamaopencarry.com/. Their first page has cites for all the court cases and AG opinions.
You might want to tell your son-in-law that, should he take the actions he spoke of, he is asking to be the subject of complaints and possible civil action.