I would try a rat that's not much bigger than the mice you've been feeding (weanling size rat - 40g or so). You don't want him intimidated by a sudden size change, and you can inch him up to small rats after he's taken a few weanlings. Use the dirty mouse bedding trick to help him recognize the prey. If he really is hungry, you shouldn't have an issue (fingers crossed!). If it works, I'd used the bedding trick for one or two more meals before just going to straight rats.