I mean.... IDK. I've turned someone down before in the blunt way I usually do (just saying "Sorry, but I don't like you back") and since they were a good friend, we eventually just laughed it off and it was all fine in the end. But my friend is different then yours and I have no idea how emotionally sensitive he is, or how he reacts to being turned down. What I want to say is that "If he really is that good of a friend, he'll be fine with just being friends" but even though that's true, I don't know how long it might take for him to be fine with being friends again. And that time in between will suck. And who knows what might happen after.
So in short, you know him best, so this is the best I could say. Maybe ask other people who are close to him, and have them help you figure it out. I guess it comes down to whether or not your willing to lose him, and for how long.
Sorry,
Andri the Arrow