Dad doesn't know

Me and my SO have been talking since February and we've known each other for a year before that. My dad didn't know of his existence because I worked with him before he went to college. My dad accidentally found out and I told my dad he's just my best friend which he is cuz we don't use titles. My dad doesn't know he's black though. I'm white. My dad told me before he's not for interracial relationships. I'm not sure if it's cuz he doesn't want his only daughter having a serious relationship, or if he actually meant it. My mom and stepdad know of him and I'm not allowed to see him again until they meet him. They found out I snuck out during the summer to meet him. They weren't mad though. I'm scared that when my dad finds out he is going to flip out....again. I'll be 18 in a few months so they can't really forbid me to see him but I don't want my dad to be mad/disappointed in me. Any advice?