I am a loving step mom, but I am at my wits end. Can anyone help me?

I have a step-son that just turned 5-year-old. He is completely out of control. He doesn't like for his dad and I to talk us to talk to each other and will often interrupt us with "Daddy ummmm..."(he doesn't have anything to say, he just says that until he can think of something to say) every time that we start to talk to each other. He won't do anything that he is told to do and will throw a screaming and crying fit until his dad tells him that he doesn't have to do it. Everyone on both his mom's and dad's side of the family says that he doesn't know that he is doing wrong even though he will tell me that he knows it is wrong. They just give him candy, put him in front of the T.V., or give him what he wants so that he will stop throwing his fit. He runs in the stores tearing them up, he hits, kicks, he was kicked out of school for destroying people's stuff, and screaming and yelling at the teacher. He cries to get his way and everyone gives into him except for me.