I'm going to ask my roommate to remove his mattress from the living room floor and return it to his bedroom
Why do you want to ask your roommate to move the mattress?
Because otherwise my room becomes too crowded with his stuff.
I commend you for standing up for yourself. It would be wise to communicate with your roommate about that.
He is being a dick to me. I'm done with this, I'm going to pack my stuff and leave his place.
If he is being stubborn and a dick about this situation, it shows that he isn't really a good roommate to live with. It might make sense to just pack up and leave because it would be super difficult to live with someone that seems to have a grudge against you. Also, it would suck to see his face everyday and you would have to stay cautious of him.