Brian had always thought of himself as a top. Not that he hadn't bottomed; especially early in his sexual life when he went mostly with older, more experienced, guys. Now, though, he was the experienced one - the one who was in control. Bottoming always seemed to be something that implied giving up power. He didn't like to be that vulnerable.
That all changed when he met Justin. He had seen through his mask of self-confidence on the first night, and that was just the beginning.
Now, when he bottomed for Justin, he enjoyed giving himself completely to another person.