I am really confused.
Dehydration does not equal yelping when getting up. I wouldn't be satisfied with that answer. He may be dehydrated by I would think (I am no expert) that even if fluids make him feel better something is not right. That could be kidney pain.
Why didn't they just take a sample from him (catheter)? Could you get a sample from him and take it in? I've never met your dog so I don't know how he is at the vet's office. Ellie is a wiggle worm and doesn't sit still for a minute. They take blood from her all the time with no sedative at all and they do it from her neck. They just hold her...