He's read the VN (and Hollow Ataraxia) and is familiar with a large portion of the rest of the Nasuverse.
He'll be able to recognise most any servant that has had a presence in another Fate series, but he won't be able to perfectly remember entire scenes word for word or the abilities of more obscure servants. So it's somewhere in-between, I suppose.
I might put a Bo3 D100 system in place with dice to determine how well you can remember certain more in depth pieces of information.
As an example, the password in German to deactivate the defenses on the Tohsaka manor would require a roll of 100.
How does that sound?

Part 1