ball python underweight and not eating

[Bogertophis]

I agree with everything you say, but just for FYI, but our female BP is 2.5 month old, 20.4" long at 97g; she grew about 1.5" since we got her last month...

The reason I'm guessing the OP's snake may be a yearling is because they got him 2 months ago in this condition- obviously starving & probably "maintenance fed" (ie. minimally fed) for some time. A snake in this condition is clearly not growing- and since it looked normal (in terms of body weight) in the picture they were shown prior to the sale, it's obvious that it didn't get this way "overnight". The big difference, compared to your BP, is that yours is eating & growing. I've no doubt that yours grew well in 2.5 months to 20"+ -that's what food does.