My boyfriend of 13 years with whom I have a 9 year old son has been cheating for 7 years?

I accidentally answered his much guarded cel and there was a woman calling - she told me right away that she is his girlfriend. He denied but she called me back and we talked for hours. She told me all the horribly upsetting details of their relationship for 7 years. That he loved her, that they had looked for home together, he had just proposed to her, he had paid her rent and their sex was amazing. I was waiting for us to get married for all these years - no hurry. He works at night in music business so lives on his own and visits me and our son few nights a week. Now I found out that all those lonely nights he has been with her. My heart is broken. I am all alone - no family here. I am from Europe. My son is sad since he has unfortunately witnessed some huge arguements between us now. He denies that there really was any relationship between them, just some occasional sex - a mistake by him. That all her stories are lies as she wants to hurt me and get him back. She sent me phone records and few other things to try to proof something. He does not even want to acknowledge the woman anymore. It has been 2 months now and I feel alone, scared and sad. I mourn the loss of the future we were going to have together as a family. I miss him physically - I love sex with him - my first real physical joy in my life. But he has been lying to me for all these years - it is so horrifying that i am just numb. I dont want to stay with a cheater and always worry, but I dont want to end it with him. I dont know what to do. So now I am back in a sexual relationship with him - as it is less scary for now. I try to talk about coming clean and proving her a lier if that is the truth, but he just gets angry and starts blaming me for listening to the crazy b...h and choosing not to trust him. He says he would never hurt me again and I am being difficult for not believing him. He is denying, avoiding and hating the whole betrayal now and I am against a brick wall. I am reading all these posts and know that everyone will tell me to dump him. I know, but I am scared. He will get mad and then there is our son!!!??? Please help, I am totally alone.