I have the same issue my ball python is going on 2 months now, but he has not even lost 10% of his body weight. A last resort would be trying to feed him a live rat or a freshly killed rat... The season could be influencing the snake as well, as the temp cools ball pythons get in the mood for snake love, this usuallly causes them to lose their appetite, make sure at night his temps dont drop lower than they were in the summer time.
I resorted to a live rat to break my ball pythons last fast.
PS Your snake feels less firm because there is no poo in him, lol, sounds weird right? But that is probably what it is, when the snake eats regulary they fill up with poo like a sock full of rocks, and you can definitely feel it as a firmness in the body of the snake, you can even gently massage the poo filled snake and can elicit a fart or two sometimes even an explosive bowel movement ! I know this because I do this on a regular basis for my constipated Blood Python, time for your weekly Fecal Massage !
(My avatar is a pic of my constipated blood python, doesn't he look like he feels violated?)
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