My Bestfriends' Checkered Past..?

Hi, this is kinda a touchy subject for us right now. Im cryin as i type..So please, keep ur ignorant comments to urself, serious answers only please. My bestfriend(a guy) and i have been friends for about 4 -5 years. we do everything together, people even thought we were dating, thats how close we were. Well, i just recently found out something HORRIFIC from his past, when he was a teenager regarding , and when i asked him if it was true, he was shocked and had tears in his eyes and said "Im Sorry.." before he could say or explain anything else, i left. well, that was a couple days ago, and hes called me a few times wanting to talk. He even came over but i wouldnt answer the door. Hes my bestfriend, & i KNOW he's a changed man, i know it was in his past, and I know he would never hurt anyone, but i am soo afraid to talk to him about it. How do i talk to him about this? How do i get over what he did then? its his apst, its over and done with, but knowing wat he did kinda scares me..