Std

[TimeSquare]

is it possible that someone caught genital herpes at some point around 20 years ago but says they don’t remember who they had sex with before they got the rash or in what year roughly they got it.

Yes it is possible, you could have it for years and have no symptoms so it could be very difficult to know who gave it to you or when. To be honest it's a pretty common thing for people to have genital herpes and it can be caught even when using protection.
As others have said, the issue is that he knew he had it and didn't tell you, that is unforgivable.