are IVF children disqualified? Legal citation on the natural born question, please.

This is "natural" in a legal context not a biological context. What this generally amounts to meaning is that you met the requirements of being a citizen when you were born. This doesn't specify a method of birth (cesarean or otherwise), but instead that you're either born to existing citizen parents, on us soil, or on otherwise recognized us territory (Puerto Rico, military bases, embassy grounds in a foreign country I think also can count) and I believe I've heard that an exception to this is that you're not being born to diplomats on official foreign government business, because not all countries recognize dual citizenship and forcefully giving the newborn US citizenship could cause diplomatic issues with the other country.

The Constitution doesn't define "natural born citizen" but the majority of scholars take it to mean "citizen by virtue of birth" rather than "citizen by virtue of naturalization."

I have no idea why you're talking about IVF children, they aren't legally different from naturally conceived children.

He is a citizen by birth, not through naturalization.

IVF is irrelevant if the biological parents are known and in a relationship, or if the birth is inside the U.S. In particular you cannot gain citizenship through donated sperm, though.