You only start with about 5 liters. I'm surprised he survived losing over half.
I am going to guess that they were giving him blood while he was still losing blood. A family member had an operation and she received 6 pints of blood and was not in a severe risk, because they were able to add blood faster than she was leaking. It just took a while to find all of the leaks.
I lost that much in a botched operation 20 years ago.
Took me 10 months to get up to maybe 75% of my enrgy back and two years before i was working out again at full speed.