Acting Out 6 Yr. Boy

I need help with my boyfriend's son. He is turning 6 in November and he will be going into Kindergarten this year. His situation is not normal at all. His dad and mom weren't married and obviously are not together. They switch the boy 2 days on 2 days off and every other weekend. I have been with my boyfriend for a year and a half now and my boyfriend's ex hates me with a passion, so I am not allowed to help out with babysitting, taking him to school, etc. But my boyfriend and his ex both work through the week till 6 or 7 at night. So during the day he is with his great grandparents who also hate me with a passion as well. When it is our time to have him we will have my boyfriends mom watch him when she doesn't work during the day. We usually have him every weekend because she is usually off drinking and partying. During the week when it is her turn to have him she will have him for 1-2 hours then drop him off at her Grandparents. She is not being the mom he needs at all, but she is so worried about me trying to take his spot. But sadly he spends more time with me than his own mother. With all this jumble he has 4 different house rules: ours, my boyfriends parents, my boyfriend's ex, and her grandparents. He always acts out so we can't take him in public or really anywhere other than these four houses. His mom is more like a sister than his mom. She lets him run all over her as well as his great grandparents. He is so spoiled with them and has literally no rules to follow. When we are not doing anything fun he says he wants to go someone else's house because he has 4 choices. He throws his tantrum and it gets horrible! He will say "I hope you die" "I will kill you" "I will leave and never see you again and you will be sad" "I will blow up your house/ truck" "I hate my life" "I want to kill myself". He shouldn't be thinking or saying this stuff I think. He got kicked out of preschool because he beat up on the girls at school and threw chairs at the teacher. He won't listen to authority and just thinks he can act out and get what he wants. He will try and hurt anyone or animals such as stepping on your toe until you say it hurts or twist your fingers till you get upset. He locks us out of the house and we just have huge power struggles with him. I'm not sure where we can go or what we can do. I believe he is too young to go into a boys home for a weekend or something but I don't know. If you have any suggestions or advice that would help us so much!