If the girl is on the top while having sex, does she do anything or does he do the work?

There's this guy at work who keep hitting on me. At first it seemed like innocent flirting but it's gotten downright vulgar and creepy. He's 15 years older than me. I'd like to remain friends with him, if possible but he seems to think that when I turn 18 it will be okay that he says completely inappropriate things in the workplace. I'd rather not file a complaint but I will if I try everything you tell me and it doesn't stop. A friend also suggested that I drop his girlfriend a line through email letting her know what's going on. I think that would be kind of mean and may even make him violent. He's really pushy so it scares me a bit. I'd like to know how to convey to him to knock it off without hurting his feelings or getting anyone else involved.