Tell your son to go to the mairie to talk to maire, secretary or whoever. We have done them over the last five years as we have replaced windows, added veluxes and so on. The maire does not actually deal with them any longer, but apparently could, so he passes them on the district. Then it comes back via him, so he signs and all is fine. On average it has taken about a month. It must be done before any work. The reason for going to the mairie is that they might not want to know or see, just let your son replace on condition they conform to local regulations (for instance, colour) but anything added, even a tiny window, they want to know but generally it is a formality and that is all. By doing it correctly it also gains the confidence of the mairie which means more significant things might be permitted faster in the future.