Sometimes they simply go off feeding for a while, sometimes for extended periods of time . For instance I have a Male that didn't eat for 4 1/2 months the beginning of this year. If your temps are right and his set up is right this "could" be the case. He will eat again when he feels like it. If he doesn't drop a noticeable about of weight there is no need to worry. You really haven't given us enough info to come to any other conclusions. Can you post a pic. of your cage set up and him? Someone could notice a problem you may be overlooking. Also what has his behavior been when you put the rat in his cage ? Does he come out of his hide because he smells it? Does he come up and lay beside it but then just go back to his hide or elsewhere in the enclosure? ..... I do a lil dance in front of them with my feeders (grab the scruff of the rats neck with the tongs and bounce it gently around about 6 inches in front of him to mimic the movement and actions of a live rat , just remember slowly .... To fast or violently and you will only scare him and then the game is over till next week