How to get my roommate to stop bullying me?

My best friend and I are roommates. Ever since we started being friends in middle school hes always tried to assert himself as in charge. And to a certain degree it doesn't bother me. Hes always bullied me a little, but it was always brotherly and not to serious. But now that we've got a place together he takes it way too far. Hes way bigger than me and whenever he sees me laying on the couch he just sits on me until its hard to breath. He gets angry if he sees me working out and he even hides my weights because he says he doesn't want me getting too strong for my own good. It used to be funny cause I thought he was joking. But its just getting uncomfortable to live with him. He hasn't really ever beat me up, so I cant be too mad... he is like my brother. But sometimes he just takes it to far, like stuffing me in his footlocker and not letting me out till I promise to make him some food or something. I dont want him to keep bullying me but I also dont want to lose my best friend. What should I do?