If he was not a full time student during parts of at least 5 months (such as, January to May) then he can't be your dependent.
If he was a full time student for at least 5 months, then he can still be your dependent if, considering his financial needs for the entire year overall, you provided more than half. That will depend on the value of room and board you provided to him (you can count a share of your rent, food and utilities--such as 1/3 the expenses if 3 people lived in the home); plus tuition you paid; medical expenses, travel and personal expenses you paid for. If you paid more than half his expenses overall, you can claim him as a dependent and he must check the box that says "I can be claimed as a dependent by someone else".
