If the birthrate is much lower than he thinks, that is a sign that child mortality is also lower than he thinks. Sweet!
Was this a mistranslation? Because the context and flow really feels like it should have been the opposite? A society with a very high birthrate, to ensure enough offspring makes it through to surviving. Or maybe it was meant to be "the child mortality is higher"/"the complement of child mortality is lower"? A society where children easily die etc.
Healthcare, sick leave, even the food situation make it hard to have a kid, and then that plus a million other things that happen in a pseudo medieval society make it hard to keep them alive, or even just keep them with the number of times child theft came up on the list under different names.
many isekai start with improving health and sanitation in the other world but I guess this protagonist didn't even think about that since he can heal anything
He took care of that stuff, but like he says in this chapter, his vision didn't extend past his slaves and family, to the family of the slaves. I don't remember the interval, but like once a month or a couple times a year he would gather all his slaves, and refresh their health with his magic, this obviously didn't extend to the townspeople he doesn't own.