I get a lot of rescued underfed ball pythons. Last year I got a 5-year-old female that was 830g in weight and SUPER skinny. She had been fed 1-2 mice per month. I started her off with one open-eyed rat pup every 5 days, then bumped her to 2 weaned rats every 7 days. Within just 2-3 months she had gained almost 400g. I bred her that season when she hit 1500g and she gave me 6 healthy eggs. Today she weighs in at 1990g, eats great, and is healthy as a horse.
Your male may not get quite that big, but with regular feeding he should rebound just fine. Be sure to get a fecal done to check for parasites and rule that out as the cause of his skinnyness.
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~* Jen *~
Pink Lady Constrictors