How do I deal with my boyfriends past?

Juliana
My boyfriend and I are two years apart. A year and seven months to be exact. In middle school, he had sex with a girl. I don't know who but he was honest with me about it. I was there for him after. I didn't like him during the time. It was only once. But now we're dating. I'm a freshman he's a sophomore. We're pretty serious. He cares about me so much. He's honestly an amazing guy who cares about his mum, his family, his friends and me. But this has been eating away at me since I started liking him. We've liked each other for almost four years. But we've been dating for almost four months. Yes I know it doesn't sound like a lot of time, but it's one of those things that just happens fast. He always puts my comfort and feelings first, he's truly an amazing guy. But how do I tell him that his past has been eating away at me? That I'll cry myself to sleep almost every night thinking about him screwing her? How do so tell him that?
Here's also some background information. I'm a freshman, he's a sophomore. We've known each other for four years. In my seventh grade, his eighth he had sex with this girl. I don't know who but all he told me when I asked about her was that she had brown hair, dyed it black and blue eyes. Every time I see my seventh grade yearbook I'll look through the girls with black or brown hair and blue eyes. Just, how do I tell him? He's been completely honest with me and has always awnsered every question I ever asked. How do I approach it?