I am a volunteer and I am hared by an employee at work?

I would like to go to medical school so I am volunteering at a local hospital. I have volunteered there for 8 months now, working 14 hours every weekend for free. There is a employee there who is constantly haring me, he behaves very rudely, making condescending comments and creates trouble for me whenever he could. He knows that I am in a relationship but constantly hares me with questions like "what's him name? Huh? What's his name?!?" and "who you emailing? why are you always emailing the same person? what's his name?!?". I've been ignoring him but the more I ignore him, the worse he treats me, now it's starting to interfere with my work. I've thought about reporting him to my supervisor, but I do not want to make trouble, plus I get the feeling he is a very unstable person. But my supervisor is writing me a reference letter for med school, and I am afraid he might try to sabotage that by making up lies about me and my work performance (he has hinted that he would do that). Everyone I work with has praised me for hard work, saying that they have never had a volunteer who put in some much efforts as me. Now with this man constantly haring me and finding ways to hurt me, I fear that all my efforts will be ruined. What should I do? I am so very worried I can't sleep at night. Please help me! :(