Gale has it right on this one. OP just to be clear however there is one assumption made,

How large is your boy?

medium rats are typically to most of us in around the 125gm area give or take. Usually fed to snakes in the 1200+ gm range. However I have seen recently 'medium' rats that are what most would call weaned or pups in the 45 gm area. This is why I question the term 'medium rat' is variable.

I too have assumed an adult snake this maybe different advise if the snake is less than 900gm. Just to confirm.