Try to understand what Jesus was saying.
Jesus said he was DEAD.
Was he physically dead? No - he was spiritually dead.
Jesus said he was LOST.
Was he physically lost? No - he was spiritually lost.
He was later found and came back to life - SPIRITUALLY.
That's the entire point of the parable.
Nothing in the story says the son quit being the son. And that is the point you are trying to make. Even though the son was separated, dead, to the father, he was still a son. This is not a picture of a loss of salvation. It is a picture of salvation.
Stranger