my 6 year old is the same way. He was already reading fluently when he started K and I'm not sure how, because he hardly ever read at home in preschool. Every once in a while I tried to bribe him to read something to me, but didn't make a big deal out of it. There just isn't a lot of motivation there.
My experience working as a reading tutor is that boys like non-fiction. Either that or super stupid fiction, like Captain Underpants or Diary of the Wimpy Kid. They don't really relate to fiction chapter-type books the way girls seem to. Maybe this changes as they get older, but for now, I am just happy to have DS read whatever. Right now in bed he is reading a book of turkey riddles from Thanksgiving. I bought him science books w/ lots of pictures for Christmas--he asked for books about other galaxies. He likes Captain Underpants. So the science books have a lot of pictures, but even though they are picture books, the content is advanced. It's actually good to get kids reading non-fiction because that is the focus on the school standardized assessments later on. Something like 75 percent of the passages on the reading assessments will be non-fiction. I also think with 5-6 year olds the attention span is lacking and that's another reason why they aren't big into books without pictures. DS doesn't like chapter books, saying they are "too long", although he will spend a lot of time reading books with pictures. I think all the print with no pictures is just intimidating. I would take your DS to a library or bookstore and let him pick. Show him the non-fiction books especially and see if any of those strike his fancy. I wouldn't worry about a "series" or chapter books. Good luck!
