Problems with a boy at work?

I work at Glaxo, a medical place where they test drugs and find treatments. I been working there sine March. I am good friends with an 19 (I am 21, turning 22 this august) name Joseph. I find him very attractive. We enjoy scaring each other. Since June, he will take me into an office and start to breath down my neck, kiss my neck and he will kiss my chest and once unzipped my jeans. He says he doesn't have time for a girlfriend but he tries to act like my boyfriend. He is still doing this. I'm afraid to report it because I have no proof and i might get fired and if i do, my mom will kick me out of the house. What can I do? I can't really talk to him because he doesn't understand English (he's Spanish and so I am) and I can't even speak Spanish well. Also, if he does and I don't, I might feel guilty. Why? Since middle school, I been called a snitch for telling on an eight grade classmate who was harassing me.