I am leaving my room with previous notice in the first week. Should I pay for the entire month?

I am leaving my room in December 7, and my roommate wants to charge me for the entire month. However, I noticed him more than 2 months ago, and just now he said that I should pay for the entire month. When I told him 2 months ago that I would leave, he said that I could pay the expenses proportional to the 7 days. I live in Arkansas. Anyone knows if I have to pay it?