In his waning years he has opened his mind to other possibilities - I am glad to have the fleeting moments I am allowed to see this side of him.
When I was around 9 or 10 I was reading for the first time Tolkien's Lord of The Rings. I had previously read The Lion The Witch and the Wardobe but I had no idea that Tolkien and Lewis had any relationship with each other.
My father came to me one afternoon and asked me why I was reading stuff about magic and the like. We talked, he would have preferred I read one of the "classics" in his mind (Mark Twain), a book of military history or better yet spend more time on mathematics. In the end he allowed me some leeway - I am glad of it.
In time I came to find and love all of c.s. Lewis' works. I simply cannot overstate the simplicity combined with profoundness that I still find to this day contained in Mere Christianity.
One must consider that Lewis was likely the most dedicated atheist in England in the early 20th Century. One evening In October 1931, J.R.R Tolkien and Hugo Dyson showed Lewis the errors of his dismissal of Christianity as a mere myth. He came to see that myth is but what has survived of the real truth.
Most folks from where I am from would openly write off the simple message Lewis explains so eloquently as watered down theology or ecumenical gobbledygook. But it is not.
One can ague, probably successfully that there is much more to the Christian faith than the simple tenets Lewis articulated. He himself did not deny this. He described this as nothing more than a hallway with doors. The hallway itself consisted of "mere Christianity", it is possible and acceptable to have doors going off the hallway so long as the baseline requirements of faith remain intact.
More on C.S. Lewis here