When I was single I was extremely squeamish about boyfriends previous sexual history, I insisted on using condoms way before HIV was considered a risk to hetrosexuals.
But I have to say to condemn a man for honesty about doing this 24 years ago is ridiculous, he has admitted he is ashamed. He is not saying "when I am single I use prostitutes because I see nothing wrong with it" in that case he would be a dump now
Years ago (more than 24) I cut the brakes on a boyfriends bike when we split up, I wouldnt dream of doing that now. Years ago a guy we met on the way home from a club invited my friend and I in for a coffee and we went (and as it happens had a cup of coffee) I certainly wouldnt take a risk like that now. I threw a drink over a boyfriend in a club, no way would I do that now.
I am a completely different person to who I was 24 years ago. All the things we have done in the past make us who we are today, some of our experiences are "shit I will never do that again experiences, hopefully this was a lesson for him, he didnt like it and wouldnt repeat it.
If I felt he were a nice boyfriend I would respect this honesty, of course if you then get other niggling reservations about him then reconsider I assume after such a short time you are not thinking marriage/babies serious life stuff. Just enjoy and see what happens...you can dump his ass in the future if he ends up not being that great.