I dont believe it's an assumption as Jesus is using the two to teach on repentance.
They would both need to change their mind. If the second did not change his mind, why did Jesus include two sons in his parable?
He could have taught only about one son.
I believe it's an assumption to say the second son did not change his mind.
Why did Jesus include the second son in the parable.
What lesson do we learn from him?
Sorry, I just deleted my post seeing that
@Hepzibah already addressed this point.
One said he'd go, but didn't, one said he wouldn't, then did. Which did what the father wanted? He didn't want them to
say something, make promises, whatever, he wanted them to
go. So the one who went, regardless of anything else, this was the one who did what the father wanted.
This is not a parable about repentance, it's a parable about obedience. You can't just stop at the story of the sons, we need to include Jesus' full answer.
Matthew 21:28-32 KJV
28) But what think ye? A certain man had two sons; and he came to the first, and said, Son, go work to day in my vineyard.
29) He answered and said, I will not: but afterward he repented, and went.
30) And he came to the second, and said likewise. And he answered and said, I go, sir: and went not.
31) Whether of them twain did the will of his father? They say unto him, The first. Jesus saith unto them, Verily I say unto you, That the publicans and the harlots go into the kingdom of God before you.
32) For John came unto you in the way of righteousness, and ye believed him not: but the publicans and the harlots believed him: and ye, when ye had seen it, repented not afterward, that ye might believe him.
John the Baptist came commanding repentance and works fit for repentance. Those who said they were the righteous wouldn't obey by repenting. Those who made no claim to righteousness, however, did obey John, by repenting of their sins and being baptized.
Much love!