How to deal with a jealous friend?

My best friend in the entire world is a guy. I see him as just a friend that I’d never date, but he definitely wants a romance with me. Every single time I even mention a guy, he gets extremely jealous and upset. He’s not my type (he’s kind of a sports kid and I’m a total nerd) and I tell him this over and over, but he’s persistent. What can I do to stop him from being so jealous?