I've been in a similar situation ie. been the guy in love with best friend who's a girl! I was 16.
What's the guy like? Is he mature etc.?
I can tell you now, that now being 18 i've learnt that, no matter what this guy does, it'll take a long time for him to get over you. It'll happen, with time, but he needs support. He needs a big BIG sign that you're just not interested.
It's difficult. Sometimes, guys think that they can make girls like them. Hell, even girls think that they can make guys like them. It's just easier for girls lol. But the fact is, you can't create attraction. It just is. It's just something that you can't describe.
Attraction is natural and hopefully this guy will realise that when you're not attracted to him, the only thing he can do, is get over you. It took an entire summer of not seeing/speaking to my best friend in order to get over her. Worked though, and we're as close as ever, and no more feelings involved anymore!
I'm not saying that things could change. Hey, you might wake up in the morning and realise you are attracted to him, and in rare cases, sometimes a guy can make a girl like them, but only in rare cases.
Hope this helps.