Our littlest DD3 is like this. She likes books with more content about more adult topics. I try to ask her what occasional words are and to try and read the titles and captions. She is not very interested in trying early readers.
I have found that she really likes comics. I think the story line in comics which is visual with short blocks of text is a good combination. The plot line in the comics are more engaging that most early readers but the words are few and the pictures also tell a story. She will look at comics and this is a good way for her to enjoy books independently and build skills on her own. She also likes us to read fiction chapter books to her.
Also our DD7 who was reading at 2.5 will still bring us things to read to her, particularly if the subject matter is very complex. I think they are able to comprehend more when they hear it and are discussing it, than when they read it alone. She also likes to listen when we read the younger one chapter books.
Our kids definitely have an interest in more complex subject matter than they can read alone. They have always been like this and I think their desire to learn more and our time constraints contributed to their early reading.
If your son really enjoys being read to and craves more complex material he will probably figure out how to read just so he can get more, if you keep reading to him and keep him interested. One day that reading switch will go off and he will probably read whatever he is interested in over night.
We have a friend who has a DD8 that is very smart and his comprehension is very high but he did not read much besides comics, then around 6.5 he went with in a couple of months to reading everything. Now it is hard to keep track of what he is reading. His mom just kept reading to him until he took over.
