Good advice to get him checked at the vets. I wouldn't worry too much. I have an adopted male BP, the previous owner was feeding him pinky mice at 7 months old! He is now 14 months old and is still small for his age (abt 400grms) but is putting on bulk quite fast now. As long as he's eating thats the main thing. Of course snakes are like people, and grow at different rates to different sizes