Do I tell my boyfriend the awful truth?

Ok, so I have a secret. I think about it everyday. I need some advice on what the right thing to do is. Here's the story: In January my boyfriend went to a party and cheated on me, he slept with some girl. They did it twice, one after the other, using the same condom. I found out about it 3 months later. I asked my boyfriend to tell me everything but I still wasn't sure he was being honest, so I decided to talk to the girl to see if she said anything different. I went on my boyfriends facebook and emailed the girl pretending to be my boyfriend. She told me something horrendous. Thinking that she was speaking to my boyfriend, she admitted that my boyfriend had gotten her pregnant and she had had an abortion at 10 weeks. I never told my boyfriend. We never mention the fact that he cheated nowadays, it's how we managed to move on and save the relationship. When my bf was 15, he got his gf at the time pregnant and she had an abortion too. It took him a long time to tell me this and he's never told anyone else. It really messed him up and he still thinks about it and it upsets him. He's never realy gotten over it. Now i don't know what to do. I've known this for about 2 months and kept it to myself. For a few reasons, like I don't want to upset him, he'll see it as having killed 2 babies. Also, I don't want to bring the subject of him cheating up again because our relationship is going good atm. Another reason is because if i tell him this i know we'll argue, and i admit that i'm a bit scared of my bf. He gets very angry. He's never hit me and I don't think he would, but he's thrown me about and pined me down and a few times i've hit my head on the wall when he's pushed me. I don't want any of that to happen obviously. I really don't know what to do. So please tell me: do i tell my bf that the girl he cheated on me aborted his 10 week old unborn child . . . or do i keep quiet and take this to the grave? Please no comments saying i should leave him for cheating, already heard all that. Thanks