Spoiled Stepson question?

I need some help. my stepson is spoiled rotten. he lived with his mother and grandmother since he was born. no male in his life and is an only child. he's now over 3 years old. we've only been married for a year. my wife went back home to finish some schooling 6 months ago and took the boy with her. It's not the kids fault but he's a nightmare. He screams, demands things in a demanding voice, throws a temper tantrum that lasts for hours when he doesn't get what he wants, won't go to bed when we ask. She visits every few months and stays about a month before the next semester starts and the month they are here, this kid gives us no peace and no time together. he's up in the morning demanding things, whinning, crying, and won't go to bed until we are tired at night or else he'll scream. i told her she needs to discipline because our marriage is suffering because the limited time we have together is shot because we can't even go out on a date alone because this child can't be more than 1 foot away from his mother. Also he's almost 3 and a half and still in diapers and still drinking from a bottle! I'm getting real tired of it. she said she's working on it but when she calls me from school and the boy is around he's either demanding things from her or screaming at her. I am asking her what the hel# are you doing? this kid is going to be a nightmare unrespectful adult. she doesn't get it. i've tried to tell her that kids need discipline and our marriage is suffering because when he's with us and we try to spend time together, the kid is so bad and undisciplined that he makes the limited time we have hell. she won't discipline or can't. I'm at my wits end. last time she was here i said next time you come home, leave him with your mother. i cannot stand hearing piercing screaming 3 times a day when all we ask him to do is take his shoes off the sofa or "no" he's not getting a new toy everytime we go to the store. He's terrible and i'm at my wits end because i really love my wife. what should i do?