In these situations, I think I would gradually start moving my sewing stuff in the room. Set your sewing machine up and just start sewing. He thinks he has one over on you because he is enjoying the room. It is not likely you will forget about it. Smart ole boy isn't he? As you move your stuff in, gradually start crowding him out or start moving his stuff out.
It sounds like the room is finished and you just need to move in with your sewing stuff. You are most patient...10 years...no wonder he thinks the room is his. Fix the issue, you will be glad you did.