Firstly, I love C.S. Lewis's books.
Secondly, I think Brandon might have taken some inspiration from Lewis, but not as a primary influence. In the postscript, he talks about how The Princess Bride was a big influence. While Goldman's book is where the story started, I do think he incorporated things he liked from Lewis's works.
Thirdly, "a fairytale for grownups" is a phrase I love, and I will end up using it as often as I can.
(p.s. FOR NARNIA!)