My ds5 does this sometimes...but he can also read "normal". Sometimes I'll see him start a book in the middle then jump ahead and back...reading paragraphs in random order. I don't worry about it because on considering it...I realize I sometimes do the same thing. Sometimes I'm just previewing a book to get an idea if I really am interested in it. Nothing like flinging yourself around in a book to see if it is interesting. As long as he can read in a normal way also, in wouldn't worry about it. Not every book you pick up is worth reading in detail. This is also kind of a speed reading technique.. Getting the gist before you jump in.
If you know he can read well, maybe it's just a quirk.
