I think you're worrying a bit prematurely... You said he fed fine for the first 3 weeks, so he hasn't eaten in a week, correct? Leave him to settle into his new set up, no handling, only mess with him when checking water/temps and cleaning.
What does his set up look like? What are the exact temps and humidity? I'd do another f/t hopper mouse on the next feeding, if that's what he's accustomed to. After he's eating consistently, you can try to switch him to rats if that's what you'd prefer.