Albert Barnes Commentary Luke 15:30

"but when this thy son came, who hath devoured thy living with harlots, thou killedst for him the fatted calf." — Luke 15:30 (ASV)

This thy son. This son of yours. This is an expression of great contempt. He did not call him his brother, but his father's son, to show at once his contempt for his younger brother, and for his father for having received him as he did. Never was there a more striking instance of petty malice, or more unjustifiable disregard of a father's conduct and will.

Thy living. Your property. This is still designed to irritate the father, and set him against his younger son. It was true that the younger son had been guilty, and foolish, and ungrateful; but he was penitent, and that was of more consequence to the father than all his property; and in the joy that he was penitent and was safe, he forgot his ingratitude and folly. So should the elder son have done.