Have you tried leaving the rat in overnight. You say that he is showing interest so perhaps giving him some extra alone time may get him going. Make sure the rat is well thawed aand properly warmed. Make sure to scent the room when you are readying the rat. You can warm it with a blow dryer and have the scent going right in the snakes tub/cage. Offer it but if he doesn't take it right away let it be overnight. Like someone else posted, use a slightly smaller prey item until he gets back on feeding regularly.