Second son - Abraham
From Sensus Plenior
Abraham was the first instance of the first-son/second-son theme that Matthew saw. The theme says that he first son loses an inheritance and/or is fruitless and is also portrayed as being earthly. The second son is the heavenly son who gains the inheritance and is fruitful.
There are two ways that this theme is played out concerning Abraham. He is literally a second son and he is figuratively a second son.
