My son live with me but he also stay in dorm and visit home every weekend .

You enter that he lived with you for the Whole Year since he is in college and only temporarily away from home.

Read the information on the Residency page when entering a dependent.

Say he lived with you all year.   Living away at school is considered to be a "temporary absence" so you can say a college student lived with you the whole year when you claim them as a dependent.