I could see a snake only being fed such a tiny amount not growing a lot, but I don't think it would completely stunt growth to the point of looking so young and healthy. I haven't had any experience with snakes on this matter, but underfed mammals don't simply stop growing to maintain a healthy body weight in the absence of food, the natural tendency to grow to a certain size range outweighs any desire the animal might have to use their little nutritional intake for maintenance and they end up bony and lanky and underweight, but usually close to normal size in terms of bone growth.

As to why the person you got him from might have been dishonest about his age, was there a chance he wanted you to think he might not get much bigger? If he really needed to get rid of it and didn't know how much you knew about ball pythons, he might have lied to try to hook you.

Also: Nice shirt.