Then he might have some sort of digestive abnormality causing this (assuming also no internal parasites...you still need to rule that out). If no internal parasites, your vet can best explore other ideas: while it's rare, some snakes have poor digestion. Years ago I had a rattlesnake that could not digest well, fats especially; it was obvious by his poor body weight (triangular shape) and oily foul-smelling stools. I first tried adding digestive enzyme powder to his f/t prey, but it wasn't enough to correct the problem, so I took a chance & let another "hot" (venomous) rattlesnake make kills for him, then I'd steal the prey before the other snake could eat them (boy did I get "dirty looks"! ) and that finally was very effective- he gained weight & his stools looked normal. (FYI, venom is a huge part of the digestive process for rattlesnakes, and the one who had trouble digesting was one that had surgery to remove his venom glands rendering him "safe"- it was NOT done by or for me, he was dumped on me as a rescue, along with some others- long story.) Anyway, I'm only telling you this because for variable reasons, snakes can have deficiencies just like any other creature. Some humans have a "weak stomach" & lack certain digestive enzymes too, & need help. Sometimes a tumor can preclude the production of needed enzymes, etc. I actually hope it's something EASY to fix like worms in the GI tract...