DON'T. You said you are best friends, and if he thinks that's what the relationship is, telling him you have let it go to a place he hasn't, could ruin the friendship. You need to ask him to tell you about this person he likes. IF IT IS NOT YOU, you have to decide if you can still leave it as friends only, or let him go completely. IT WON'T WORK BOTH WAYS. THIS is why it's so hard for men and women to just be friends.
Don't say anything at all. If you are the one he's into he'll tell you when he's ready. Bringing it up beforehand, especially when you are not sure yourself, would just be adding more awkwardness to an already strange situation and could end up driving him away, destroying any friendship you already have
I'm not an adult so don't @ me but
Be honest about your feelings. Keeping them hidden will be torture (and you never know if he'll like you back). If you have a real conversation and talk about it, and he's a good friend, he should understand that your friendship doesn't have to end. There'll still be hope for a relationship, but from personal experience, especially if you were already close friends, it'll feel normal to be just friends eventually.
You don't have to end a good friendship for something like this!
Hope I could help, even if just a little :)