This exact same thing happened to one of my foster puppies. She tested negative for parvo, but it seemed to be exactly like parvo. She was losing proteins too and I think her white blood cell count was almost nothing. She actually had to have a plasma transfusion. I really thought she was going to die She ended up staying in the emergency room for 4 days, but was able to go home after that and made a 100% recovery

Hopefully Toby will recover quickly and this nightmare will become a distant memory.