Sorry... Lizards are my specialty... Not rodents... If you ever need to force-feed an iguana, or you have a lethargic bearded dragon, I'm your man!
But seriously, I'm sorry about your beloved pet... I would just make him comfortable, and continue to give him the things that he likes. If possible, you should separate him from the rest of your rats. Try to move him to a slightly warmer area, to help keep him out of shock, so he can pass on in a comfortable manner.