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I'm assuming this is a young/hatchling snake that you're working with, so a live rat that's a correct size isn't capable of hurting him anyway - you're just terrorizing the rodent unnecessarily. I would get a live rat pup and just leave it in with him overnight. Once he's eaten a few good times for you, you can start the tough love of switching him to F/T, if that's the preference.
Younger rats are easily the size of adult mice, and much less dangerous, hence why none of mine ever see a mouse if possible.