S.D.
What about validating his feelings? It sounds like everything is coming from YOUR point of view: rewards, talking telling him it won't happen again. He seems to need to be afraid about it for a while and saying "There's nothing to be afraid of." doesn't resonate with what he's feeling inside.
Saying, "You're afraid the roof is going to fall" Then, listening to what he has to say about it. "That was very scary wasn't it." Even acting the whole thing out with him, so he can process the fear until he's done. Making a blanket fort and going inside and then knocking the roof down (it's safe and soft) and then, you acting out your part by saying "oh, you are so scared." instead of "it's okay don't be scared" may help. because obviously he is scared, and it isn't okay.