In short, I disagree with Keller's interpretation of the father. The father does what any compassionate father would do. He has to let the boys make mistakes, but he longs for a restored fellowship with them. I cannot see the mixing of attributes of the sons into the father. The father is not reckless like the younger son is. He is showing love, letting the boys make their mistakes, and restoring fellowship upon repentance. Nor is the elder son righteous like the father. The elder son shows by his actions at the end that he is not righteous. He expects his works to be enough.
The younger son is reckless. The father is righteous. The elder son is a legalist. I can see describing the father as extravagant but not reckless. The father is also strong enough to forgive when wronged, which the elder son is not.
I have a different reading on the parable than most writers do. Let's look at the three actors in the drama.
The Elder Brother
I take the stance that both boys are in the wrong. The older brother is not strong. The elder sits back and ignores his family duty to mediate the dispute. This duty is explained in Brad Young's The Parables, 141. Referencing Kenneth Bailey's, Poet and Peasant and Through Peasant Eyes, Young states:
When the younger son asked for the inheritance, the responsibility and obligation of the elder son was clear to the first-century listener. The father should have been told to leave the matter in the hands of the elder son, because the younger boy did not really mean what he said or realize how much such a request would hurt his father. The elder son should have demanded that his younger brother apologize to their father. As mediator, the elder brother could have sought reconciliation between his brother and their father.
[I can see that as striving to honor his father in the light of keeping the younger son from dishonoring the father or having the younger repair the dishonor he has committed.]
However, he sits back and inherits the double share. As there were 2 sons, the money would be split three ways with the eldest receiving 2 shares.
His concerns at the end of the parable are for himself.
Luk 15:29 But he answered and said to his father, 'Look! For so many years I have been serving you and I have never neglected a command of yours; and yet you have never given me a young goat, so that I might celebrate with my friends;
30 but when this son of yours came, who has devoured your wealth with prostitutes, you killed the fattened calf for him.' [NASB]
We know from the end that the elder views his father as a banker. He does not love his father or brother. He is a greedy opportunist. Most think the elder is innocent until the son returns. He is not. As he inherited double, he had reason to stay silent. He was expected to intervene and keep the family together. He is obedient to the letter but disrespectful of his father long before he humiliates him by not going to the banquet. At the confrontation, he does not refer to his father by an honorific. He refers to his brother as “that son of yours.” All his relationships are broken. He will not join the community who came to celebrate the younger's return.
The older son is legalistic. He is missing the relationship with the father just like the younger son. Both sons see the father for his money. Neither sees him for a parent. The father is there throughout the story, ready to reconcile. One is far away in body; while the other is far away in heart.
Nothing in this parable would offend or really surprise a well-intentioned, thinking Jew. Reconciliation between God and man is the central point of Judaism. (Jesus never attacked Phariseeism, He only called out the hypocrisy of certain Pharisees. Indeed, Christianity owes much to the Pharisees. One of my seminary professors, Wave Nunnally, an expert in 2nd Temple Period Judaism said, "Christianity is like a stew of Judaism. Lots of meat and broth from the Pharisees. Essenic and Zealot spices mixed in.")
The Younger Son
When the younger son asks for his share, he is saying he wants the father to die. When the father splits the amount, he retains control of it as long as he lives (Mishnah Baba Bathra 8:7; cf. BT Baba Bathra 136a; Sirach 33:19-23; BT Baba Metzia 75b). This is something like a living estate. The elder son gets a double portion of the estate. Notice he says nothing when his brother leaves. Both sons lack relationship to the father. The younger son sells his inheritance and leaves. As the father kept possession until he died, the son probably had to sell it at a discount. The son goes as far away as he can and wastes the money. When a famine hits the land, he is left destitute. His friends were fair weather friends. He gets a job feeding pigs (the lowest a Jew could go) and even they have better food then he does. Pigs were fed carob pods in those days. R. Acha said that Israel would never repent until it was forced to eat carob (Leviticus Rabbah 35:6). The boy comes to his senses and desires to return home. He repents. He recognizes his sin and sets out to go home. He prepares a speech that includes how he has sinned against heaven (heaven is a Jewish circumlocution for God). He is ashamed of his wrongs and wants to pay back all he has wasted. He knows he has sinned against God as well as his father. When he returns, his father does not even allow him to finish his speech. He gives the son a ring (probably a signet ring meaning the son now had access to the family finances), a robe, and sandals.
The father loves his sons enough to let them make their own decisions. Jews would have immediately linked the father to God. Notice how Jesus casts the all powerful God as the helpless parent. God allows people to choose even when they make the wrong choices. Like a parent, God will give all to bring His children into a right relationship. A father must never be too proud to accept his children. A son must never be too ashamed to return (Dt Rabbah 2:24).
In the Intertestamental Period, "The Father" or "Abba" became a name for God. At Qumran, it was the preferred name for God. There are rabbinic parables that portray similar situations. "The Runaway Son (Pesiq. Rab. 44)" and "The Obstinate Son (Midrash Devarim Rabbah)" both involve runaway sons and a compassionate father. See also the story of Honi the Circle Drawer who called on God to send rain to "your children" and his prayers were answered (Mishnah Taanit 3:8). His successor, Abba ben Honi, did similar though explicitly referred to God as Abba.
Abba ben Honi had just entered a village in need of rain. The local teacher told the children that only Abba could bring rain and they must beseech him to do so. The children see b. Honi and run to him. "Please send us rain, Abba!" He looks on them with compassion, raises his hands and eyes to heaven and says, "Abba, Abba, send rain to your children who cannot tell the difference in the Abba who can send rain and the Abba who cannot." [Thanks to Bruce for locating it as BT Taanith 23b.]
The Unending Story
Parables put the listener on the spot. They are to put themselves in the story and see where they fit in. In this story, the elder son does not restore fellowship. We don't know what happened. Jesus leaves it hanging. The elder was expected to go in and at least pretend to be pleased his brother had returned home. Instead, the father goes outside and they have a heated discussion. Jesus is putting the decision in the listeners hand. They are the elder brother and must end the story for themselves by deciding how they will treat those who return to the Father.