Is it only that horse or is it any horse that enters the room? To me it sounds like he isn't paying attention. He's more focused on that other horse then to you. A suggestion might be to work with both of the horses together, go for a hack in a field or just exercising together. See what their like in the pasture, do they completely ignore each other, fight, etc.
You must maintain your horses control. Work with him everyday if you have to. Do Serpentine's and figure eights, whoa and back up. Just do basics with him . He should than realize that your the boss, not him.
Good Luck :)