When I first got my male bp he was almost 2 years old and only 250 grams! Its was shocking. He wouldn't gain weight, even eating mice every week. He never refused food. Then I switched him to rats and whoa those saved me a vet visit. He he hadn't started gaining weight w rats I was going to get him checked for parasites. But now 8 months later he is 1090 grams! So there is hope!! Feed rats! & keep hoping. If not get the bp checked for Parasites.
Good luck!!