What do I do?

So I just turned 17 and since I was 16 my step dad has been slapping my ass every time I walk past even I tell him to stop he keeps doing it. So today ran and slapped my ass when I was walking to my room and I told him can you not. Then I went down stairs when he was outside and told me my mum if she could tell him to stop she said I have to do it and have an adult conversation with him about not slapping my ass. But I don’t want to because it will make me to uncomfortable and I already feel like throwing up. I know it’s sexual assault but I dong think he realise it