Can my son (my dependent) avoid being subject to kiddie tax

My son is 23 and graduated college last May.    He had a part time job in 2017 with only 6,000 in income.  he sold some stock in 2017 that he purchased with his own money and had a cap gain of 4500.   turbotax forced him to have to pay the "kiddie tax" based on my income.  is there any way to avoid this since it his his money that had the actual gain