Why must the younger son go back to the Father in the parable?
The younger son must return because he realizes he has nothing in himself and no other source of sustenance. Only the Father, the Giver of covenant promises and spiritual life, can meet his need. So he goes back to the Father.
Les explains that the younger son, like the one little lamb caught in the thicket, realizes his need and that there is only one place he can go for sustenance—back to the Father. He must return to the Giver of covenant promises and spiritual life. In himself he can do nothing. This parallels Jesus’ parables, which showed the Jew their state and their need for Him. So the son goes back to the Father.