How can I convince him I'm not interested?

I dont think you should try to convince him of that. You already told him the truth, and he'll think that either you lied the first time (and what would that make you, telling a guy with a girlfriend you're attracted to him?) or that you are lying now, in which case he'll still know you're attracted, and whichever he believes, he'll know one answer was a lie. Dont be a liar, act like a grown-up, who can be friends with someone she also finds attractive, without it being a big deal. It happens all the time, and dealing with it is part of life. Just be a good friend, dont lie or fake anything, and you'll be fine.