My DS got the reading fluency by playing Gameboy games that required reading, such a Pokemon, and Monster Ranchers. There are a few on the YSP list who always come forward that their sons learned this way.
It makes sense to me that as a child's working memory is occupied with sounding the word out, comprehension will drop - what doesn't drop is the urge to get information at their readiness level - so it can be a tough transition. Think of a child learning to type (ahaaa, a sweet thought) - there will be a tricky time where they need to practice, but won't really be able to do much with the skill, yet....
It may help to let your son know that reading will get more useful, and that practice is required, but the end is worth it.
Smiles,
Grinity
