Losing 10g in 11 days is not a good sign if he is a baby. Especially combined with runny, really smelly poop. Sounds to me like parasites. How much did he weigh before? Maybe someone else with more knowledge on BPs can chime in on it but using the guideline for boas as well as bearded dragons which I have both of and have had awhile, losing that much weight in such a short time assuming the animal is only a couple hundred grams to start with as well as the runny, smelly poop is warning signs of parasites and probably a vet visit.