Augustine of Hippo

But already planted in the Church, he begins to confess his sins, nor says he all that he promised he would say. For it follows, And his son said unto him, &c. He wishes that to be done by grace, of which he confesses himself unworthy by any merits of his own. He does not add what he had said, when meditating beforehand, Make me as one of thy hired servants. For when he had not bread, he desired to be even a hired servant, which after the kiss of his father he now most nobly disdained.

Catena Aurea by Aquinas ↗

Bede

And the son said to him: Father, I have sinned against heaven and before you. I am no longer worthy to be called your son. Now he begins, established in the Church, to confess his sins, and he does not say all he had promised to say, but only up to this: I am not worthy to be called your son. For he desires to become through grace what he confesses himself unworthy to be through merits. He did not add what he had said in that meditation: Make me like one of your hired servants. …

On the Gospel of Luke ↗

Bonaventure

Second, with respect to the reverence of the son accusing himself he adds: And the son said to him: Father, I have sinned against heaven, namely through contempt of the Majesty presiding over all things: Job 11: "It is higher than heaven, and what will you do? It is deeper than hell, and how will you know?"; and 22: "Do you not know that God is higher than heaven?" etc. …

Commentary on Luke, Chapter 15 ↗