Hmmm. This is an interesting POD. Having a son born in 1573 would definitely change things. For one Henri wouldn't be able to divorce Marguerite. Everything else depends on how this son, lets call him Henri, is raised. If he stays at Court with his grandmother as a hostage for good behavior, which I imagine is most likely, then he'll no doubt be Catholic. Next, with one son we could end up seeing more from the Navarre Couple. Not likely sense they would have "got lucky" on the first try, but possible. Third, assuming the butterflies don't change anything, like Charles IX's daughter is born a boy or Louise of Lorraine has a child, things will gt even more interesting once uncle Henri III dies (If everything goes the same its not unreasonable to think there would be assassination attempts and that one could succeed) the question becomes what next? Do we follow the line of succession and make Henri of Navarre the Protestant King or do we bypass him and make Henri the Younger King directly? I think that question is gonna be a major one to answer.