My 6 year old son is showing negative behavior and I dont know how to help him!?

I have 6 year old boy/girl twins. They have seen way too much in their young lives. They witnessed there dad coming home from work always in an angry mood, not able to deal with them. He loves the twins but his abusive in every way childhood, in my opinion, does not know how to deal or handle with them. By the time the twins were 4, their dad and I separated and were divorced. They seem to adjust with him out of the house okay, but I know it has a huge impact on them. Since the separation my ex husband takes the kids every other weekend. The twins have already met 2 of his girlfriends. They told their dad they did not want to sleep over the girlfriends home but on his weekends he took them their anyway. My son is allergic to cats. The first girlfriend owns a cat and my son would come home with allergies so bad he ended up in the emergency on a nebulizer. I took my exhusband back to court to have the visitations away from the home. Needless to say that relationship didnt last much longer because the ex girlfriend didnt want to get rid of her cats and decided she didnt want to date someone with children. The new girlfriend seems very good with my twins but my son does not like her for some reason he wont explain. Their father moved in with her almost immediately after the first relationship ended. My daughter on the other hand really likes the new girlfriend. I have a male friend that stays here alot but I tell the twins we are friends and thats it.They dont see any romantic, emotional relationship between us and I think that is why they really like my friend who is like family to us and has done more for my twins than their own dad. My son has become very disrespectful talking back to people. He is lazy and does not want to pick up after himself, do homework, has no patience to listen when someone is explaining to him right from wrong. The worst part is my ex husband is always behind in child support so I never have money for them to go out and have fun. But my ex husband picks them up takes them out with his girlfriend, buys them toys all the time. Last weekend I punished my son for hitting his sister and yelling in her face. Their dad came to pick my daughter up to go to breakfast while I kept my son in his room. When my ex came back with my daughter I explained to him what is going on with our son. My friend said to my ex that my son is very disrespectful. My ex got so mad that my friend got involved he just gave him an attitude and went to my sons room. When they came down I asked my ex if he sent the child support to the DOR because he was a month behind and we had no food in the fridge and my bills are not getting paid. Needless to say the conversation escalated into a screaming match infront of our twins. I just kept telling him to make a payment so I can get food shopping done. My ex infront of the twins were calling me money hungry, psycho crazy *****, hugging my twins saying your mother is crazy look how she is yelling. I was beside myself. I did get mad and yell I am to blame also. But never calling names just telling him to pay child support. Now my son is talking very angry to the sister. He is definitely acting out. I sat with them and tried to explain to them its not their fault and it is a grown up thing. And how sorry I was to lose control and let them witness any fighting. My son was good for 2 days, but now again he is acting up hitting his sister, not wanting to get ready for school, he is always bored and doesnt want to do homework, only wants to eat junk food, doesnt want to listen or be told what to do. My daughter is very responsible, mature, happy and loving little girl. I know that their is something I can do for my son or both children to get them on the right path to a healthy life. Unfortunately I dont know how to go about it and I really need advice. These kids are all I got. I cant stand to know if they are in pain because of their parents stupidity. I am trying to open up an in home daycare to make more money so I dont have to depend on their dads check and I can be home for them. I just dont want to see these two go through anymore and I want to see them both happy and learning and making great childhood memories..