Please help save Leo

After a couple days of lethargy and loss of appetite, my beautiful Leo, who saved my son from a house fire, took a turn for the worse on Christmas Eve, and was hospitalized with suspected hemorrhagic gastroenteritis (HGE). Things have been a rollercoaster with each step of progress being nullified by a setback. Vets at the emergency hospital have done blood tests, ultrasound and a urinalysis. But things just aren't progressing as they liked. He's just under 6 years old. A rescue from Texas. And three years ago, when my son was home alone with him, Leo raised the alarm of a fire downstairs in the kitchen. He saved my son. I can't just give up on him. But the bill for his care has now exceeded $10k and is set to grow more. The emergency Vets are out of options, and they've now referred us to an internal medicine specialist for an endoscopy and biopsy. We're in desperate need of help.