(101) So We gave him good tidings of a forbearing boy. (102) And when he reached with him [the age of] exertion, he said, "O my son, indeed I have seen in a dream that I [must] sacrifice you, so see what you think." He said, "O my father, do as you are commanded. You will find me, if Allah wills, of the steadfast."
فبشرناه بغلام حليم فلما بلغ معه السعي [As-Saffat 37:101-102]
The pronoun in “reached” refers to the boy, and the pronoun in “with him” refers to Ibrahim. As for “sae” (labor/effort), it may mean earning a livelihood or it may mean walking. Both are correct, but the closest meaning to me is that it refers to walking, for “sae” is frequently used to mean walking, as in the saying of Allah the Exalted:
O you who have believed, when [the adhan] is called for the prayer on the day of Jumu'ah [Friday], then proceed to the remembrance of Allah
يَا أَيُّهَا الَّذِينَ آمَنُوا إِذَا نُودِيَ لِلصَّلاةِ مِنْ يَوْمِ الْجُمُعَةِ فَاسْعَوْا إِلَى ذِكْرِ اللَّهِ
[Al-Jumu'ah: 9]. Likewise, Allah the Exalted said:
And when he turned away, he strove in the land
وإذا تولى سعى في الأرض
So the meaning of “sae” is walking. However, the word “with” implies companionship, for he had become a follower of his father, walking with him. He was not a small child who stayed in one place, nor a grown man who had become independent. A small child in a cradle does not reach the stage of walking with his father, and a grown man who is independent does not walk with his father because he is on his own. But this boy had reached the stage of walking with his father and was constantly in his company. This is the age at which a father’s attachment to his son is strongest, for the soul does not become attached to a small child in a cradle, nor to a grown man who is independent. Rather, it becomes attached to one who is of this age. This is part of the wisdom of Allah (Mighty and Majestic) in that He tested Ibrahim (peace and blessings be upon him) with this clear trial.