"And Rebekah spoke to Jacob her son" (Genesis 27:6). But was Esau not also her son? Rather, because Jacob would obey his father and his mother, as it is said, "And Jacob obeyed his father and his mother" (Genesis 28:7); but Esau would obey only his father. Therefore she called him her son.
Why Scripture Calls Jacob Rebekah's Son
Midrash Aggadah, Genesis 27:6
ורבקה אמרה אל יעקב בנה. וכי עשו ג"כ לא היה בנה, אלא שיעקב שומע היה אל אביו ואל אמו, שנאמר וישמע יעקב וגו' (בראשית כח ז), ועשו לא היה שומע אלא לאביו, לפיכך קראתו בנה: