And I think that's a challenge I don't think it's warranted to take up and try to do. I think God in the judgement will tell you it was absolutely self evident by the nature of the parable that the Son had freedom of will to choose, that is leave and come back. Not to say I won't have to give account in the judgement for a great many things for none of us are perfect but this will be one for Calvinists.
Let me say that the parable of JESUS about prodigal son brings a great mystery if analysed by the own Word of GOD. Paul said to the Church of Corinthians, chap. 1:v.27-29:
27 But God hath chosen the foolish things of the world to confound the wise; and God hath chosen the weak things of the world to confound the things which are mighty;
28 And base things of the world, and things which are despised, hath God chosen, yea, and things which are not, to bring to nought things that are:
29 That no flesh should glory in his presence.
JESUS revaled in the parable the existence of two sons, two brothers, and both were /are heirs of the property of the Father, GOD the Father.
Two sons - two inheritances
JESUS revaled in His parable the existence of two sons, two brothers, and both were /are heirs of the property of the Father, GOD the Father.
The main heir - the firstborn is the main heir
We have to remember that the firstborn is the main heir, therefore, the inheritance of the younger son is different of his older brother.
Both are legitimate sons, and legitimate heirs
Generally speaking,
all of God's sons are adopted, according John 1:v.11-14, with the exception of the two children mentioned in Jesus' parable about the legitimate prodigal son and heir to a part of the Father's property.
Who is the legitimate younger God's son, younger brother of JESUS?
Being JESUS the firstborn and legitimate GOD's Son, and also the main Heir as the firstborn:
who is the younger brother, also heir of a part of the Father's property?
He exists, JESUS was not talking about a legend, a folkloric fantasy, but a hidden reality, a fact, a mystery. The glory of GOD is to conceal a thing: but the honour of kings is to search out a matter-
Proverbs 25:v.2.
So, Who is the legitimate younger God's son, younger brother of JESUS? By the way, he has not genealogy, but JESUS has.
There are three that bear record / testify in heaven
(heaven has nothing to do with sky): The Father
(GOD the Father), the Word
(the Word made flesh-JESUS - the Word is GOD), and the holy Spirit
(who is not a ghost as is written in English language, but a Person) and these three are ONE
-1John 5:v.7, and John 15:v.26, and 16:v.7-15-Take a look.