וְיִשְׂרָאֵל אָהַב אֶת יוֹסֵף מִכָּל בָּנָיו כִּי בֶן זְקֻנִים הוּא לוֹ "And Israel loved Joseph more than all his sons, because he was a son of his old age" (Bereshith 37:3). As we know, Onkelos translates 'a son of his old age' as meaning 'a wise son to him'. We are told that Joseph was able to absorb every drop of Torah that his father transmitted and that is why he loved him and made him the coat of many colors.
It is difficult to understand how Israel (Ya'aqob Abinu, 'a"h), was able to transmit all his Torah to his son Joseph. After all, Ya'aqob Abinu, 'a"h, studied the entire Torah for 14 years, night and day at the Beth Midrash of Shem and 'Eber. This is in addition to all the wisdom he had acquired before going there at the age of 63. How was it possible to transmit all this knowledge to Joseph in 4 years. He started teaching him at the age of 13 and Joseph was sold into slavery at the age of 17.
What this teaches us is that Joseph's wisdom was so great that he was able to fully grasp, in this short period of time, everything that his father transmitted to him. This is the explanation of Onkelos' translation of 'a wise son to him'.
(See Dibrei Mordekhai, Parparaoth, Parashath Wayyesheb)
Divrei Mordechai, Dibrei Mordekhai, Vayeshev, Vayesheb, Wayyesheb, Parasha, Parsha