What Shoild I Do? Please Help Me

I am 31 and have been married for 5 years. We have a 4 yo son. My husband has angry outburtsts when he yells at me, calls me names. Everything can set him off and has become a nightmare for me to live like this. He hit me in the past (not fists, but he slapped me, pushed me, spat at me, etc). These episodes are more and more frequent and uglier. Today he yelled at me because he claims that I spend all the money-which is not true. He left with our son and when he returned he bought him presents. I told him what was the need to buy him another round of gifts as Santa already got him. I was surprised that he complained about the money and then went out and bought toys. He told my son something like" I told you she will be upset if I buy you presents". I asked him why he talks to our son like this. Than he started with the yelling, threathened me with his fists. I remained calm I told him to stop it and leave me alone. But he started to slap me all over my face, my head. Our son was watching and got really scared and I said to him to calm down, that everything was going to be all right. Then my husband started to acuse me that i threatened my son with a knife. I said "what????" He started recoring him and kept asking him to say I threatened him with the knife. This is not true, this is such I lie. I did not do such a thing!!! My kid was really scared and he said that I threatened him. Husband asked him to hit me, which he did. He bit me and left my arms all bruised. He bit me until I bleeded. I didn't touch him as I was afraid my husband could say that I hit him on purpose. I had to leave the house alone eventually. My God what happened???I am still in shock and please excuse my writting but I am still shaking with fear. I just can't believe how bad things are. It is so sad that it breaks my heart. He turned my little boy against me. He tells him I not a good mother and he shoulnd't love me. Is this a sane person?Who would do such a thing? I really don't know what to do. I have a job and a place to stay, but I just cannot make myself strong enough to file for divorce. It is just so hard. And my mind always tells me what if, what if I had been better, what if I set him off...I hate these what ifs! I hate the lousy person I have become. I don't know what should I do, just feel so lost and lonely.