Thanks for your responses. It sure helps to talk with folks that have been through this. I have always had dogs but never been through Parvo. And I had always been told it was fatal, so to be honest, if these pups had been mine at the time, I would probably have let the vet euthanize the one I decided to keep. I was fostering him when he became sick & started vomiting, so I immediately took him to the vet who wanted to euthanize. I told him I would contact owner to see what he wanted to do. The vet gave the male pup IVs under his skin, & I brought him back home with me. I could not return them to their owner for fear of infecting the litter mates. By the next morning, the puppy was so near death that I did not think he would make it through the day. He could not raise his head, & blood came out both ends. I fed him Pedialyte mixed with catnip tea every 15 minutes and constantly changed his puppy pad. I cleaned with a strong solution of Clorox hoping his litter mate would not get sick. But she did.
I did not take her to the vet since I knew he would want to euthanize. I treated her the same way I did her brother. But I had ordered Parvo Guard from Amber Technologies which came overnight. I started her on it before she started showing symptoms, and she did not get as sick as the male puppy. He was deathly sick for 2 days. His body was lifeless and cold. He finally drug himself far under the bed wanting to be left alone. So, I figured he was crawling away to die. I cried in despair and gave up on him; remarkedly, the next morning, he was up scratching at the door, whining. I cried like a baby!!! Then through the day, he would pester his sister wanting to play. I had to separate them so she could get rest and recuperate. They both survived, but I had to wait until the vet thought it was safe before I took them to their owners. But the guy that was supposed to adopt the male did not seem to reliable and was reluctant to pay his vet fee. So, I kept him. I had not named them since they were not my dogs. So now, the male is stuck with Mister for a name. We called them Mister & Sister, but the female's owner had a name picked out for her.
Until I received your replies, I had never considered that his bulk might be the reason he tires easily. He is huge. But he lost more weight than the female pup did. I had worried about the high fever he had with Parvo damaging his brain and his heart. But, it is obvious that nothing is wrong with his brain. He is amazingly intelligent. He brings me his dish when he wants to be fed. When he finishes eating, he brings it to me to be put away. He sits on command, shakes hands, lies down, almost rolls over (he hates that one) and almost stays until I give the command to come to me. (He hates that one too). He is trained to lead with a halter but hates a collar. He is great with other dogs and cats and kids, so he could adapt to nearly any situation. But he thinks he is my protector. We have such a strong bond, and I am glad the owner did not seem to really want him.
I have talked with the owner of his litter mate. She says Puddles is very smart too. So, I don't think the fever harmed their brains. I guess time will tell if any damage was done to their other major organs.
Thanks again for the responses. All advice is appreciated & might help Mister's progress.