Parvo was the first thing I thought of when I first started reading your post.
Our Angus had it when he was a puppy and got very, very sick. We thought we were going to loose him. But our big guy pulled though. He's had his bout with being sickly, but now is a healthy 12 yr old 106# rottie boy
I wish your pup had been able to pull through it too. What a sad way to go.

The people here at PT are very kind and loving, if you need to talk we are here.
Welcome to our wonderful community.
Anna