My six year old is like this. He can decode words even if he has no idea what the words mean. So if I gave him a college textbook he could read it and make it sound somewhat fluent but would have no idea what he's reading.
The answer is to give books at a level where the kid understands what the words mean. If they don't have the vocabulary, they are not going to understand the book. Pictures help because then if the kid doesn't understand a word, they can sometimes figure it out from the picture. But if there are too many foreign words, they will quickly lose interest.
DS is not particularly strong in verbal ability (probably borderline "gifted" at best) so comprehension is third-fourth grade if he reads to himself. I don't go any higher than that even though he could technically read something that's a higher level.
He is more of a math/non-verbal kid than verbal. I think he uses his non-verbal "pattern recognition" skills to decode but they also have to be strong in verbal ability in order to comprehend well above their age.
