'The Father does not interrupt his younger son. Instead the prodigal changes his mind and in a moment of genuine repentance surrenders his plan to save himself and lets his father find him. He comes finally to acceptance of being found.' So writes Kenneth Bailey in his wonderful study of the prodigal son story.
We often see the younger son's repentance happening while he was far away from home. Jesus says 'he came to himself' and we have understood this to mean that he recognized that he'd screwed up and needed help to sort himself out. So, he'd go home, confess his mistakes and seek mercy.
This is not what's happening, However. Rather the son has calculated that he could work his way back into his family's affections and acceptance in the wider community by going home, working for his father, learning a trade and earning the wherewithal to repay his debt. And so off he goes.
But the father's response derails the younger son's plan. His idea of working his passage back to acceptance through hard graft is derailed by his father gracious welcome, his kiss and embrace. So in Luke 15:21 the son changes his speech and drops the idea of working for his dad.
Why? Because he realises that 'repentance' is not a work we do, it is an acceptance that we cannot do anything but accept God's offer of being found.
If you think about it, the father didn't 'find' the younger son in the story. Rather the younger son finds his own way home. Yet his father keeps talking about finding him. He was still lost until the second before his father embraced him. At that moment he was found. The issue he had to face was 'am I prepared to be found?' It's a vital issue because his pride was at stake.
Good Christians believe in salvation by grace through faith. But we like to cling on to our repentance story, the story that says we had the good sense to get it together with God, the story that narrates how we worked out that Christian faith was true and worthy of our allegiance. And in that story is the salvaging of our pride. We can't contribute to our salvation, so we talk about how we worked our way to faith. Repentance becomes the work that puts us in a position of being accepted by God.
This was the view of Jesus' audience - especially the pharisees for whom he composed the three linked stories that comprise a single parable in Luke 15 - as Bailey shows. It was not Jesus' view. For him repentance means accepting that we've been found by the God who comes looking for us - symbolised in the story by the shepherd, the woman and the father (all pictures of God drawn from the OT and all pictures of the ministry Jesus is doing - see Luke 19:10).
So everyone in his audience - women, sinners, pharisees, scribes, poor, rich, whatever - can be found because Jesus, who is looking for them, is there with them. The question is, do they want to be found? Will they accept being found?
This is why the story doesn't end as we expect it to. The older son is also found in the parable, but will he accept it? We don't know because the story ends before it reaches its conclusion. And the reason or this is simple: every hearer of the story is like the older brother; every hearer has to decide 'do I want to be found? Am I going to accept being found by Jesus?'