Personally I'd keep him where he is for now- his body, as best I can tell, has good weight, muscle tone & good proportions (if you imagine a "cross-section" a boa should be shaped like a loaf of bread). He surely doesn't appear thin, & while he could probably put away a medium, he might end up packing on excessive weight- I'll always vote for "slow growth". He's a handsome fellow, & I like the cooperative way he's perching on your scale.When you do finally go to medium rats, you'll also want to lengthen the time between meals- they take longer to digest- but I wouldn't rush it. He looks great.