If he's 511 grams he's big enough to take a small rat. If he's been underfed then there's no concern with transitioning - he'll likely grab whatever feeder is offered.
I've had rescued underfed BP's take rabbit weans and small chicks. Most snakes in captivity are overfed, it's just ball pythons are one of the few species that will self-regulate so they don't end up obese.
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