While I think that they can probably tell if a prey item is spoiled or bad based on the scents that are coming off of it, I find it hard to believe that a ball python in the wild would do very well if it was in the habit of turning its nose up at meals that weren't perfectly healthy (though I could certainly be wrong). It would be an interesting ecology research project, offering wild balls sick mice to see if they would refuse or eat. I wouldn't want to do it with captive animals for fear of making them sick. Anyway, just keep trying and I'm sure he'll eat eventually. With frozen as opposed to something that you bred or even just live from somewhere, you can't really tell the quality of the item. Have you ever considered starting your own breeding colony? As for f/t when they don't eat, I would only refreeze a prey item once, as the more time it spends thawed out the more that bacteria will build up. Good luck getting him eating.