Bad tempered step son

I am a mother of 4 , my kids are aged 13,11,6 & 11 months and my partner has a child who comes to stay every weekend who is 6 , for the past few months I have had to deal with his child calling me loads of nasty names his child shares a room with my child of same age when he is here but recently I had to take my eldest child’s room away and make him share with the younger one so that my partners child has his own room because when he has his really bad temper tantrums he starts to smash up the room and has tried to rip wallpaper off the walls and my radiator last week his temper tantrum lasted for nearly 10 hours straight ( no joke) the child’s mother spoils him rotten and always rewards his bad behaviour with toys and sweets and now she has started trying to put her foot down his behaviour has become really bad I am at a loss as what to do when he is here and anything his dad says to him has no effect and then his child just says ” I don’t care “ and then it’s a load of name calling from the child , I worry about my youngest child around him because he isn’t very nice I caught him dragging her around the room by her vest he Has hit her in the face with a teddy bear and a blanket because he had taken them out her hand and I asked him to give them back and he through them at her nastily I am looking for some advice on what to say to my partner or just any advice in general would do my kids get brought up to respect people and follow the rules and if they don’t there is consequences and I am worried that his child’s behaviour is going to have an effect on how my kids behave please help 🙄🙄🙄