Thanks for sharing. My father read The Hobbit to me when I was about the same age as your son (~5 years old). I absolutely loved it and, when I was older, read LOTR on my own (still remember being mad that Bilbo wasn't the main character anymore), which started a long and interesting journey throughout the fantasy genre.
I'm sure the Harry Potter series would serve as a great starting point as well.
"When the going gets tough, the tough get empirical." -- Jon Carroll