Well, the guy I'm currently dating started off as my best friend. I really thought he was just that, a friend, until one night he finally admitted he really liked me, and that not telling had been eating him up for months. I didn't really know what to make of it at first, and 3 months later, which we had spent growing even closer, he kissed me. And then, I finally saw what I really felt for him, and how great we'd be together. So far, it's going perfectly well.
Now, the point of this isn't to flaunt my cheesiness, it's to tell you that the worst thing you could do would be not telling him. Sure, it'll probably be one of the hardest things you'll ever have to do, but if you don't, it will consume you. The worst thing that could happen would be if he said he didn't feel the same way. Then what? At least, knowing this, you could move on. And who knows, maybe he'll change his mind. I know I did. Besides, I know lots of people who were mutually attracted to each other, went out together, and both realised after a month it just wasn't working out, and they're still good friends. Maybe you'll see that dating him isn't like what you imagined.
My advice would be, tell him when it's just the two of you, possibly try not to have any plans with him the day after! Don't freak out about it, just tell him you'd understand he didn't feel the same way, but you had to tell him anyway, and see what comes of it. When my friend told me he liked me, it made me reconsider everything, and knowing he liked me made me open up to the idea that maybe, I did too. Just tell yourself it's not that big of a deal, and maybe that will help.
Hope this was any useful!