Not firstborn, First Son in the same way as First Lady.
Neither one of them, “son” or “lady” (small L) are worth much. No one in the media has ever referred to Eric or Don,Jr. as “First sons”. And certainly not to Barron. But whatever. Hunter Biden is a junkie, a worthless debris and criminal. Like a lot of DC clowns working their federal grift.
Is the First Lady the wife of the president or the White House hostess?
Occasionally, you’ll see Jefferson’s daughter(s), Dolly Madison (under the Jefferson Administration), Buchanan’s niece, et al. listed among the First Ladies.