Have you tried an alternate food source? We never really keep venomous snakes here but if it's like any other what about a gerbil or hamster? What about scented pinkies? How is the environment it is in right now? Are you sure it's at optimum for him? Does he have a nice tight hide spot? Have you taken into account temperature drops? Brumation?
Questions, questions, questions....
Also, you might think to put this in the Venomous Snakes section of Fauna instead of the General Herp talk. Might be able to catch someone there that doesn't check this forum category.
Good luck,