Boas go through a "growth spurt" around 6 months to 14 months or so. They will eat like pigs, if you let them. All the boas I have were only on mice before I got them. A boa outgrows mice quick and could easily be on rats their whole life. Mice always seem to digest a lot faster than rats do, so it makes perfect sense to me for him to be hungry after a few days. Try the rats and see how your luck goes. A good rule of thumb is to feed your boa,especially when it's younger, a day after it poops.