Any Advice?

So, we've got this thing with Sam.

Apparently, he has a real problem with enclosed stair wells. We found this out on Christmas eve when we were delivering snack baskets to a local hospital. On of our stops was in the basement, and the easiest way to get there was via a stair well. As soon as we opened the door and he looked inside - not even being near it - Sam went into a meltdown. That little boy was absolutely terrified.

Mike stayed upstairs with him. He said it took a good while for Sam to calm down. We wondered about it, discussed how bizarre it was, then pretty much forgot about it.

Until last week.

We took a quick trip to Tennessee (what a story that is) and our rooms were upstairs. There were two staircases, one inside, one out. The evening we got there, Sam didn't want to go up the inside stairs, but Mike coaxed him up. The next day, however, when we were leaving for lunch, he freaked out again. He was hysterical, terrified. The only way we could calm him was to take him down the outside steps.

So that's what we did. We always had to use the outside stairs with Sam. The very thought of him having to use the enclosed stairs sent him into more hysterics. And he wasn't just being difficult. He was honestly scared.

So, my question is, how do we help him overcome this fear? Deliberately exposing him is something we are unwilling to do. His fear is real and he is so upset. I don't want to do that to him on purpose. That's just cruel. He has no problems with elevators or open or outside staircases.

Thoughts? Suggestions?