Gonna need a little more info before we can be much help. Can you describe his enclosure setup? Glass tank? Plastic tub? How big? How are you heating it? How are you measuring temps and what are the temps? What's the humidity in the tank? What are you using as substrate? What about hides? How big/old is the snake? How big were the mice the previous owner was feeding?

If you'll provide this info, we can make suggestions on adjustments that might get your snake going. If he's only gone three weeks without food, its really not panic time yet. They can easily go for several months without food.

Steve