Sometimes it's the brand of the mouse too. I have 1 young snake who will ONLY eat mice from Layne Labs but refuses (completely and totally) from any other source, even home raised (and I tried all of the tricks in the book).
Note: it may not hurt to find out the band of rodent he had been previously eating. It looks like he's been eating well at some point so this honestly may be the issue, or might work.
I've been getting the mice from the same place i bought him. the people who run the store breed and sell the rodents in the store so i know its all from the same source