Abraham worries only about Isaac when he exiles Ishmael in Genesis 21:11

When Abraham is compelled – at Sarah’s behest and with God’s approval – to exile Hagar and Ishmael, the Torah tells us (Genesis 21:11)

וירע הדבר מאד בעיני אברהם על אודת בנו
But the matter was very bad in Abraham’s eyes on account of his son

The universal assumption is that the son referred to here is Ishmael. I respectfully disagree and maintain that the son about whom he is worried is in fact Isaac. The reasons are both textual and observational.

From an observational standpoint, if Abraham had been at all sensitive to Ishmael he would have been no less sensitive to Hagar, and the verse would have read על אודתם (on their account) rather than על אודת בנו (on account of {only} his son) .

This would have made perfect sense for someone like Abraham who could muster sympathy even for the Sodomites. Hagar is here a total innocent; a woman who has borne him a son. If Abraham had no choice but to acquiesce to Sarah’s diktat surely his moral misgivings should have covered Ishmael’s mother as well.

What’s more, Abraham doesn’t quite seem all that concerned about the welfare of either Hagar or Ishmael. Wealthy man that he is, he could do a lot better better that push Hagar and Ishmael into the desert equipped with only a jug of water and a loaf of bread. He could provide them with a few camels., a servant or two, a generous purse, and enough food and drink to help them reach a safe haven. Clearly, however, this is not the case. He simply engages in a classic Arab divorce by showing Hagar the door and slamming it shut. Hence ascribing to Abraham even a scintilla of compassion on Ishmael rings hollow indeed.

But now let us examine the textual evidence.

In the first ten verses of Chapter 21 Yitzhak is referred to as the son of Abraham multiple times.   Yet when Ishmael is mentioned he is referred to only as the son of Hagar, not of Abraham. Indeed, even in the single verse that makes it clear that Ishmael is Abraham’s child , the Torah states this obliquely. Instead of using the term Ishmael his son ישמעל בנו the verse says:

ותרא שרה את בן הגר המצרית אשר ילדה לאברהם מצחק
And Sarah saw the son of Hagar the Egyptian, who she had borne to Abraham (21:9)

By contrast, when it makes the filial connection between Yitzhak and Abraham it expresses this directly:

ויקרא אברהם את שם בנו הנולד לו אשר ילדה לו שרה יצחק
And Abraham named his son who had been born to him, Isaac (21:3)

וימל אברהם את יצחק בנו
And Abraham circumcised his son Isaac… (21:4)

ואברהם בן מאת שנה בהולד לו את יצחק בנו
And Abraham was a hundred years old, when his son Isaac
was born to him
(21:5)

The fact is, Ishmael is no less Abraham’s son than Yitzhak, yet he is referred to only as the son of Hagar:

ותאמר לאברהם גרש האמה הזאת ואת בנה
כי לא יירש בן האמה הזאת עם בני עם יצחק 

And Sarah said to Abraham, “Drive out this handmaid and her son, for the son of this handmaid shall not inherit with my son, with Isaac” (21:10)

Clearly neither Sarah, nor Abraham, nor even the Torah itself is willing to classify Ishmael as Abraham’s son proper.

Hence it stands to reason that when verse 11 tells us of Abraham’s misgiving regarding “his son”, it can only be Isaac he is worried about.

So what exactly is Abraham fretting over?

There has been much psychological research concerning the trauma caused by the loss of a sibling, especially in childhood – even if this loss predates the surviving child’s birth.  The long-term effects can range from mild neurosis and insecurity to outright emotional breakdown up to and including a desire to die because of a perceived unfairness which deprives the surviving sibling of the right to live.

(Parenthetically this may explain Yitzhak’s readiness to submit to the akedah without any hint of resistance.)

Abraham is fully aware of the potential trauma to “יצחק בנו” once Isaac becomes aware not only of the loss of his older brother, but of the fact that he was not lost to illness or accident, but rather to having been cast out by their shared father.

And surely Abraham understands very well what this awareness can do to his own relationship with Yitzhak. After all, what child would ever trust his father, knowing that he had sent his older brother to a virtually guaranteed death.

Hence it makes perfect sense that :

וירע הדבר מאד בעיני אברהם על אודת בנו
But the matter was very bad in Abraham’s eyes on account of his son

Abraham is no fool. He understands that actions have consequences. He grasps the overarching significance of sibling love. He realizes that what he is about to do – no matter how justified or beyond his control – will have long term consequence, hence:

וירע הדבר מאד בעיני אברהם על אודת בנו