Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
No doubt that mixing with opposite sex which is widespread in universities is forbidden. May Allah protect Muslim men and women from it.
Therefore, your son should seek a college which is free from mixing. If he finds such a college then he should Praise Allah and continue his education in that college. If he does not find such a college, and as long as he needs to complete his education, he may continue his education, but he should lower his gaze, guard his chastity and keep away from females as much as possible. Also he has to keep company with good male friends who could help him to lower his gaze and stay away from Haram acts.
Allah knows best.