You may go through a few rats but you can always go the wait it out route and just monitor his weight when all else fails when they get hungry enough they will eat. Just watch for major weight drops eventually he will accept the rat if not and he starts loosing to much weight then you may have to stay on mice.
Best way I have found to scent is to remove mice from their housing if live and put rat in and let them trade houses for a couple of days so that the scent sticks. If that doesn't work try making him wait it out they usually will not go more than a few weeks at most a month or so without eating if his weight is good you shouldn't see much of a weight drop