A2A, In my view, there’s nothing anybody can really tell you that will either a) increase the probability of him saying yes, or b) make it so your friendship will just go back the way it was before you asked him. That all depends on him, and we don’t know him.
I would also caution against the probably false belief that it will just “evolve into something great” all on its own. It’s possible, but not at all probable, because relationships / friendships develop inertia and it just gets harder and harder for either person to make a decisive move. If something is going to happen, it needs to happen while the feelings are still fresh.
My suggestion to you would be to simply tell him how you feel about him, try to be patient with his reaction, and let what happens happen. Feelings seem like things to tip-toe around, but all that does is promote confusion. You don’t know how many fights I’ve had happen over misunderstandings.