Ok first of all, he is NOT still with her. Trust me. There are a billion reasons why they broke up, and I know he would never want to go back with her. AND my best friend is semi-friends with one of my boyfriend's ex's friends, and I know a lot of the things that are happening right now. They are not together.
Second of all, the reason he continued a relationship with me while he was with her is because he TRIED to break up with her, but she would not let him. She even threatened to kill herself if he left her. He was basically trapped. And with his son in the picture, he couldn't just up and leave. (Do you see now why he does not want to tell her about me?) He didnt have enough money for that, and he was paying for a lawyer - the breakup was happening while he was trying to get custody of his son. I do understand this and no one will change my mind how I feel about this.
Third, I was very suspicious of him in the BEGINNING of the relationship. I was concerned that he would cheat on his girlfriend of 5 years (what made me think he wouldn't cheat on me? blah blah blah), so I did some things to make sure he wasn't doing anything. From time to time I WILL check, because the trust, I don't think, will ever be fully there.
Fourth, yesterday I did find out that his son did open his mouth and now she knows. She is now calling me non-stop and calling me a b*tch and whatnot. I asked her why she has such a problem with it and why she can't just move on, she said she didn't want to and was hoping that some day he would realize what he lost and come back to her. I can see now why he didn't want her to know about me, she really has issues with herself that she needs to take care of with a counselor or something.
Thanks everyone for the advice. I did have a long talk with him last night about his sister not knowing and stuff. I think things are solved, and I'll be meeting her sometime during the holidays. So things are good now.