Step parenting Terrible 4 year old ?

My husbands ex-wife recently sent us my 4 year old step son to live with us for the next 8 months. Basically she is choosing her new husband over her children who has made it clear that he can't handle the children in the house that aren't his. Anyhow, the child arrives here and everything was okay for the first 2 weeks and then have been down hill since then. The first few weeks he was a little hyper, little whiney but I thought he would get over it like he has the last times he has visited. He is constantly whining about everything he is almost 5 and still wets and poo's his pants every night and goes as far as to openly take off his pants during the day and pee on the carpet and he knows how to use the restroom. I also have a 11 year old and 1 year old. My 1 year old was a preemie she is quite small for her age and we have explain to him countless times he has to play nice with her and not be rough (things he should know he has a 2 year old sister at his moms house). He is always beating on my 11 year old which she can defend herself and walk away but my 1 year old can not. He has pushed his sister off the couch and jumped from the top of the couch onto her, that is just one example of things that occur countless times a day, I can't even go to the restroom without taking the baby in there with me of fear he will seriously injure her. He will not sleep at night, he gets out of bed atleast 5-7 times of night to get things fromt he fridge or cupboard. Which brings me around to the wetting the bed at night, he is waking at night frequently I can not understand the wetting. He eats just as much as the other kids if not more so I know he isn't waking because we arent feeding him enough, i have even went as far as making sure we push dinner time later in the evening. He has hit me I have bruises the size of baseballs on me. He lies like a pro, I know adults that can not lie as good as him. He covers one lie with another lie and is the best back peddler i know. You can talk to him forever about the things he does and he will say ok ok ok ok, but nothing ever sinks in he goes right back to doing it 5 minutes later. He acts like this when his father is home but its a little less worse than when he is not here. His father does deal with the issue so we arent disagreeing on anything. I don't know what to do, my husband works all day and I am a stay at home mom so I am here with the children. This is stressing me out, I spend most days at nap time crying cause I can not deal with his anymore. Any advice would be helpful.