Ramban

AND WE SAID UNTO MY LORD: THE LAD CANNOT LEAVE HIS FATHER; FOR IF HE SHOULD LEAVE HIS FATHER, HE WOULD DIE. Rabbi Abraham ibn Ezra explained this to mean “and his father would die.” But if so, Judah would have said: “Our father cannot leave his son, for if he should leave his son, he [the father] would die.” Or he should have said, “We cannot bear that the lad leave the father,” for they would not make the plea for compassion for their father dependent upon the lad, [saying, as the verse has it. ‘The lad’ cannot leave his father for if he should leave his father, he would die], since they considered him as a child who did not know the difference between good and evil. [Therefore, if it be as Ibn Ezra says, i.e., that the concern in this verse is lest the father die, they would have said, “We cannot bear that the lad should leave his father,” or “Our father cannot leave his son.”] Rather, the meaning is: The lad cannot leave his father on account of his youth and his being the darling son in the lap of his father who loves him, and if he should leave him and come on the journey the lad would die.