I am not judgmental in any way towards you, believe me. I see a strong woman in bad situation, and you are right that in most cases we are not given all of the information. There are usually three sides to every story, your side, their side and the truth as each person perceives an experience from their own perspective.
You said "He has been using my money and he took my bank card everywhere with him. If I confront him about anything at all he leaves - for 4 or 5 days at a time and I can't contact him. The last time he left he contacted me after a few days and asked me to meet him. Unfortunately I was furious by that time as he has done this so many times, and I really yelled at him. I discovered that he is living in a room on his own. Now he says it's over. But he is working now and says he will put money in my account. I feel so bad for my son and I don't know what to do."
Here's a man who took money from a woman and her child, he took control of your bank card, he disappears without notice to do who knows what. He got a room without telling you, and has put you in debt. Your child may not be close to him as you are telling me now, but whether you realize it or not this person has an effect on him, and effect that may not be apparent until he's an adult and may do the same to his wife.
You said, "He has told me that he doesn't want me to stay with my husband as he believes he is cruel." and "My son is 8 and has told me that he just wants it to be me and him. His relationship with my husband was good but not deep as my son never trusted him not to leave." Children sometimes have a better handle on being straight out and honest about things.
You're right, I don't know the whole situation, I can ONLY respond to what you tell me and give suggestions based on that. With that being said, I never meant to infer that you weren't a good mother, you were very clear that you love your son and only wanted what was best for him and I commend you for that, children need all the love and support that they can possibly get.
As for you reuniting with your husband, if that's what you want to do, by all means do it. Do try and keep in mind what has happened, and if you cannot get into why he did what he did to you and how it can be prevented from happening again, then it will most likely happen again. Also, you might want to have him pay back your money by setting up some type of payment plan each week where you can put that money away for your son, or in case something happens.
Again, I certainly did not mean to imply that you are selfishly following some type of destructive path, and it wasn't meant to ti you over the edge, but to cause you to look at in a clear manner from a pragmatic point of view, from someone who is not emotionally invested in the situation, particularly when you speak of violence. Too many women are being killed in situations where the man is violent and goes over the edge, sometimes its a mistake, sometimes not, but even a small amount or once in a while violence is absolutely unacceptable, especially with children in the home.
I welcome your thoughts, let me know if you want to talk more.