So, has he gone completely OFF food, or is it just taking him longer to take the prey?

If he hasn't, just sounds to me like he's mildly stressed about losing his old hides. He should adjust and start using them within the next week or so. Kind of like a toddler having a tantrum after having his/her favorite toy taken to the Sally Ann. Hah.

It's when he refuses food completely that something needs to be done.