I used to tutor literacy and one thing that really helped new readers develop fluency was duet reading. You and your son read outloud together, with you running your finger under the words as you read them. You go as a comfortable pace, not slow, not fast. If he stumbles over a word, you just keep reading -- allowing him to join back in. (Obviously, he needs to have the basics down and be able to read most of the words for this to work.) This takes the pressure off of decoding EVERY word. Select passages that very interesting to him and easy to read, but don't ask any comprehension questions or draw attention to parts that are difficult for him. With my adult students I worked up to about 15 mintues at a time, with a young child I would start with something very short -- maybe just 5 minutes. Both of the people I tutored really enjoyed this at the end of the lesson.
It is just something to try and see if you DS enjoys this kind of practice. Different things work for different people