I think you might be!I may be a lone voice bleating in the 19th-century wilderness (as usual), but I think rumours of the demise of elder/eldest are much exaggerated.
So you'd say his oldest son, would you, Alx? To indicate his heir?
To be honest I hadn't taken the context into account, I was replying as if it were the case of talking about anyone's son (or heir).I thought that speaking of the Kings of Judah I might be granted a bit of licence. I get precious little of it, even at Christmas.
I can think of no better way of showing the distinction between oldest and eldest.... Kings of Judah who have sacrificed their eldest sons - the dearer to them and to their interests, the more important the sacrifice and hence the more likely to propitiate the god.
I wouldn't want to say their oldest sons, because I want to stress that these boys were their father's heirs, and eldest son seems the clear way of doing that...