Black Pine regurge syndrome

I've had this four year old male black pine het for pied for 4 freakin' years with regurge syndrome. Tried everything including stool transplantation from healthy black pine. Fecals always clean. Tried flagyl many times despite the clean fecals. Tried "probiotics for snakes," which is pretty much BS. He can eat exactly one pink rat (with skin peeled back for easier digestion) every four days. anything more and it's yak or partially undigested stool. anything with a bit of fur on it... yak. he's about 4 1/2 feet long and looks like a rail but is always hungry. Many times, I just thought about euthanizing him but did not have the heart to do so. I don't brumate him because he's too thin to survive the brumation and eats throughout winter.