How old is he?

He doesn't look obese to me. If a ball python is obese, you can see the skin between the scales when the snake is lying in a straight line. Yours seems to be a healthy weight. He might have been a bit skinny when you got him.
I found this chart to be quite helpful ( http://livingartreptiles.tripod.com/id108.html ). Just keep in mind that every ball python grows at a different rate, so yours might be a bit smaller or bigger.