Christian scholars are not saying that the term "son" does not occur.
What they are saying is that it is set in the Gospel narrative as a prediction of Jesus' rejection by the Jews and subsequent crucifixion, for Matthew 21:45 states:
When the chief priests and the Pharisees heard his parables, they knew that he was speaking about them.
Neither Thomas or any other evidence on hand claims Jesus declared that this parable was about his Passion. The idea that it, along with the other parables of Matthew 21 as foretelling the crucifixion, and with the Jewish leaders responsible for instigating it no less, is not found in the earliest sources.