Snake with neuropathy- I really need help!!!!

Ok, here's the story. I just got this Kenyan sandboa shipped to me UPS overnight. He was nice and warm when we got him, and well packed and insulated. But we knew something was wrong when we took him out of the bag. He has almost no muscle tone, can barely hold his head up, and has trouble moving himself forward. His righting reflex is weak. We have 20 other sandboas and I've never seen anything like this. We called the vet and got him there in under two hours. After looking at him, the vet said there was definately a neurological problem. He pointed out lumps on his body that were formed by his GI tract being partially paralyzed. The only symptom the snake really has is this flaccid paralysis, no convulsions, no stargazing that I've seen, no mouth rot, no respiratory infection. He was sent to me with some retained shed. The vet says he thinks it's either IBD, paramyxo virus, bacterial encephalitis, or poisoning. The seller says the snake was just fine before he was shipped, and says its either shipping stress or some chemical that was on the truck with him.
Have any of you guys ever seen a sand boa with these symptoms? Or any snake? Is there a blood test I should have the vet run? The seller did offer a refund if I want to ship him back if he's not better soon. I'm a little worried about the ramifications of some of these diseases on the seller's collection, but right now I have no proof of what the snake is sick from. I almost wish he'd let me euthanize the snake and still give me a refund. This is just a mess, at any rate, I'd appreciate any advice.

Erin and Jason Benner