need some help

[Goochie]

Hi everyone i've just got a Burmese python off someone that wasn't looking after it properly so i've now got it. I've had a burm before so i know what i'm getting myself in to but i had it from a hatchling and it was always a good eater from day one. The problem with the one i've just got is that it's only been feed baby chicks and it won't eat anything else. Has anyone got any ideas how i can get it to eat small rats as it's 3ft and want to start fattening it up a bit as it's a bit skiny any advice would be appreciated thanks