The best way to gauge is his body condition. Is he plump and round ? Or does his spine show prominently making him look triangular ? If he's just a touch thin, that can easily be fixed with a few good meals. Unless he is scary thin, it's nothing to worry about. Some ball pythons are long and lean, some are short and fat. Both types can be completely healthy. I have a long, lean female. She's mature and healthy, but compared to some of my others she looks terribly thin. I also have a girl who is a good 12 - 14" smaller than the others I own the same age, but she is round and thick. She'd get overweight very easily if I wasn't careful.

For the most part, body condition is the most reliable method of judging health.

Gale